To reproduce: 

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load 
    Me.CustomersTableAdapter.Fill(Me.NwindDataSet.Customers)
    Me.RadGridView1.DataSource = Me.CustomersBindingSource
    Dim view As New ColumnGroupsViewDefinition()
    view.ColumnGroups.Add(New GridViewColumnGroup("Customer Contact"))
    view.ColumnGroups.Add(New GridViewColumnGroup("Details"))
    view.ColumnGroups(1).Groups.Add(New GridViewColumnGroup("Address"))
    view.ColumnGroups(1).Groups.Add(New GridViewColumnGroup("Contact"))
    view.ColumnGroups(0).Rows.Add(New GridViewColumnGroupRow())
    view.ColumnGroups(0).Rows(0).Columns.Add(Me.RadGridView1.Columns("CompanyName"))
    view.ColumnGroups(0).Rows(0).Columns.Add(Me.RadGridView1.Columns("ContactName"))
    view.ColumnGroups(0).Rows(0).Columns.Add(Me.RadGridView1.Columns("ContactTitle"))

    view.ColumnGroups(1).Groups(0).Rows.Add(New GridViewColumnGroupRow())
    view.ColumnGroups(1).Groups(0).Rows(0).Columns.Add(Me.RadGridView1.Columns("Address"))
    view.ColumnGroups(1).Groups(0).Rows(0).Columns.Add(Me.RadGridView1.Columns("City"))
    view.ColumnGroups(1).Groups(0).Rows(0).Columns.Add(Me.RadGridView1.Columns("Country"))

    view.ColumnGroups(1).Groups(1).Rows.Add(New GridViewColumnGroupRow())
    view.ColumnGroups(1).Groups(1).Rows(0).Columns.Add(Me.RadGridView1.Columns("Phone"))
    view.ColumnGroups(1).Groups(1).Rows(0).Columns.Add(Me.RadGridView1.Columns("Fax"))
    RadGridView1.ViewDefinition = view

    RadGridView1.BestFitColumns(BestFitColumnMode.AllCells)
End Sub

Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles MyBase.Shown
    Dim spreadExporter As New GridViewSpreadExport(RadGridView1)
    Dim fileName As String = "..\..\exported.xlsx"
    Dim exportRenderer As New SpreadExportRenderer()
    spreadExporter.SheetName = "Sheet1"
    spreadExporter.ExportVisualSettings = True
    spreadExporter.SummariesExportOption = SummariesOption.ExportOnlyTop
    spreadExporter.SheetMaxRows = ExcelMaxRows._1048576
    spreadExporter.HiddenColumnOption = HiddenOption.DoNotExport    
    spreadExporter.HiddenRowOption = HiddenOption.DoNotExport
    spreadExporter.FileExportMode = FileExportMode.CreateOrOverrideFile
    spreadExporter.RunExport(fileName, exportRenderer)
    Process.Start(fileName)
End Sub

Workaround: by using the SpreadExportRenderer.WorkbookCreated event you can remove the header cells and add new ones in order to display the same header layout as in the grid:
http://www.telerik.com/help/winforms/gridview-exporting-data-spread-export.html
http://www.telerik.com/help/winforms/spreadprocessing-working-with-cells-insert-remove-cells.html
http://www.telerik.com/blogs/getting-started-with-radspreadprocessing-volume-1

To reproduce:

public Form1()
{
    InitializeComponent();

    ColumnGroupsViewDefinition columnGroupsView;
     
    columnGroupsView = new ColumnGroupsViewDefinition();
    columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
    columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
    columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
    columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());

    columnGroupsView.ColumnGroups[0].ShowHeader = false;

    columnGroupsView.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow());
    columnGroupsView.ColumnGroups[0].Rows[0].ColumnNames.Add("colPINNED_LEFT");

    columnGroupsView.ColumnGroups[1].Rows.Add(new GridViewColumnGroupRow());
    columnGroupsView.ColumnGroups[1].Rows[0].ColumnNames.Add("colDATE");
    columnGroupsView.ColumnGroups[2].Rows.Add(new GridViewColumnGroupRow());
    columnGroupsView.ColumnGroups[2].Rows.Add(new GridViewColumnGroupRow());
    columnGroupsView.ColumnGroups[2].Rows[0].ColumnNames.Add("colTITLE");
    columnGroupsView.ColumnGroups[2].Rows[1].ColumnNames.Add("colTEXT");
   
    columnGroupsView.ColumnGroups[3].Rows.Add(new GridViewColumnGroupRow()); 

    columnGroupsView.ColumnGroups[3].Rows[0].ColumnNames.Add("colPINNED_RIGHT");

    this.radGridView1.ViewDefinition = columnGroupsView;

    this.radGridView1.Columns["colDATE"].Width = 110;
    this.radGridView1.Columns["colTITLE"].Width = this.radGridView1.Width - 181;

    this.radGridView1.TableElement.ViewElement.RowLayout.InvalidateRenderColumns();
    
    columnGroupsView.ColumnGroups[0].PinPosition = PinnedColumnPosition.Left;  
    columnGroupsView.ColumnGroups[3].PinPosition = PinnedColumnPosition.Right; 
    
}

Workaround: do not set the ShowHeader property to false. Use the ViewCellFormatting event to hide to necessary cell borders to simulate cells merging.

To reproduce:

ColumnGroupsViewDefinition columnGroupsView;
 
columnGroupsView = new ColumnGroupsViewDefinition();
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());

columnGroupsView.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[0].Rows[0].ColumnNames.Add("colPINNED_LEFT");

columnGroupsView.ColumnGroups[1].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[1].Rows[0].ColumnNames.Add("colDATE");
columnGroupsView.ColumnGroups[2].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[2].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[2].Rows[0].ColumnNames.Add("colTITLE");
columnGroupsView.ColumnGroups[2].Rows[1].ColumnNames.Add("colTEXT");

columnGroupsView.ColumnGroups[3].Rows.Add(new GridViewColumnGroupRow()); 

columnGroupsView.ColumnGroups[3].Rows[0].ColumnNames.Add("colPINNED_RIGHT");

this.radGridView1.ViewDefinition = columnGroupsView;

this.radGridView1.Columns["colDATE"].Width = 110;
this.radGridView1.Columns["colTITLE"].Width = this.radGridView1.Width - 181;

this.radGridView1.TableElement.ViewElement.RowLayout.InvalidateRenderColumns();

columnGroupsView.ColumnGroups[0].PinPosition = PinnedColumnPosition.Left;  
columnGroupsView.ColumnGroups[3].PinPosition = PinnedColumnPosition.Right;

To reproduce:
- Add ColumnGroupsViewDefinition and subscribe to ColumnWidthChanged event.
- Start the application and resize a column.

Workaround:
public class MyColumnGroupRowLayout : ColumnGroupRowLayout
{
    public MyColumnGroupRowLayout(ColumnGroupsViewDefinition view) : base(view)
    {
    }

    public override void ResizeColumn(int delta)
    {
        base.ResizeColumn(delta);
        //This method will be called when a column is resized
    }
}

class MyColumnGroupsViewDefinition : ColumnGroupsViewDefinition
{
    public override IGridRowLayout CreateRowLayout()
    {
        return new MyColumnGroupRowLayout(this);
    }
}

How to reproduce:
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill
    }
    private DataTable GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Age", typeof(int));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "Name " + i, i + 10, DateTime.Now.AddMinutes(i), i % 2 == 0 ? true : false);
        }

        return dt;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        this.radGridView1.Columns[1].Expression = "Id * Age";
    }

    private void radButton2_Click(object sender, EventArgs e)
    {
        string s = "default.xml";
        SaveFileDialog dialog = new SaveFileDialog();
        dialog.Filter = "xml files (*.xml)|*.xml|All files (*.*)|*.*";
        dialog.Title = "Select a xml file";
        if (dialog.ShowDialog() == DialogResult.OK)
        {
            s = dialog.FileName;
        }
        this.radGridView1.SaveLayout(s);
    }

    private void radButton3_Click(object sender, EventArgs e)
    {
        string s = "default.xml";
        OpenFileDialog dialog = new OpenFileDialog();
        dialog.Filter = "xml files (*.xml)|*.xml|All files (*.*)|*.*";
        dialog.Title = "Select a xml file";
        if (dialog.ShowDialog() == DialogResult.OK)
        {
            s = dialog.FileName;
        }
        this.radGridView1.LoadLayout(s);
    }
}

Workaround: iterate the columns prior to loading the layout and remove the expressions
private void radButton3_Click(object sender, EventArgs e)
{
    foreach (GridViewDataColumn col in this.radGridView1.Columns)
    {
        col.Expression = "";
    }

    string s = "default.xml";
    OpenFileDialog dialog = new OpenFileDialog();
    dialog.Filter = "xml files (*.xml)|*.xml|All files (*.*)|*.*";
    dialog.Title = "Select a xml file";
    if (dialog.ShowDialog() == DialogResult.OK)
    {
        s = dialog.FileName;
    }
    this.radGridView1.LoadLayout(s);
}

To reproduce:

for (int i = 0; i < 5; i++)
{
    this.radGridView1.Columns.Add("Col" + i);
}

ColumnGroupsViewDefinition columnGroupsView = new ColumnGroupsViewDefinition();
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());
columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup());


columnGroupsView.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[0].Rows[0].ColumnNames.Add("Col3");


columnGroupsView.ColumnGroups[1].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[1].Rows[0].ColumnNames.Add("Col2");
columnGroupsView.ColumnGroups[2].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[2].Rows.Add(new GridViewColumnGroupRow());
columnGroupsView.ColumnGroups[2].Rows[0].ColumnNames.Add("Col0");
columnGroupsView.ColumnGroups[2].Rows[1].ColumnNames.Add("Col1");

columnGroupsView.ColumnGroups[3].Rows.Add(new GridViewColumnGroupRow());  
columnGroupsView.ColumnGroups[3].Rows[0].ColumnNames.Add("Col4");

this.radGridView1.ViewDefinition = columnGroupsView;

this.radGridView1.Columns["Col2"].Width = 150;
this.radGridView1.Columns["Col0"].Width = 200;


Workaround: either set the columns width before setting the RadGridView.ViewDefinition or call the TableElement.ViewElement.RowLayout.InvalidateRenderColumn after specifying the width.

To reproduce:

public Form1()
{
    InitializeComponent();
    DataTable dt = new DataTable();
    dt.Columns.Add("Col@", typeof(string));
    dt.Rows.Add("Unit");
    this.radGridView1.DataSource = dt;

    this.radGridView1.EnableFiltering = true;
    this.radGridView1.BestFitColumns();
}

Workaround: modify the Name of each column in order to remove the "@" symbol:

foreach (GridViewColumn c in this.radGridView1.Columns)
            {
                c.Name = c.Name.Replace("@", "");
            }

Workaround: before printing set the AutoSizeRows = false, then you can set it again to true

private void radButton1_Click(object sender, EventArgs e)
        {
            if (!this.radGridView1.Columns["ImageColumn"].IsVisible)
            {
                int height = this.radGridView1.TableElement.ViewTemplate.Rows[0].Height;
                this.radGridView1.AutoSizeRows = false;
                this.radGridView1.TableElement.RowHeight = height;
            }

            this.radGridView1.PrintPreview();
            this.radGridView1.AutoSizeRows = true;           
        }

To reproduce: 
1. Drag and drop RadGridView
2. Open Property Builder
3. Add a column and select it 
4. Open Settings tab and you will see that there are two ampersand symbols 
5. Add template or select Master Template and you will see that there are two ampersand symbols

To reproduce:
- Set the grid properties as follows:
    radGridView1.MasterTemplate.AllowAddNewRow = false;
    radGridView1.MasterTemplate.AllowDeleteRow = false;
    radGridView1.MasterTemplate.AllowDragToGroup = false;
    radGridView1.MasterTemplate.AllowEditRow = false;
    radGridView1.MasterTemplate.EnableFiltering = true;
    radGridView1.MasterTemplate.EnableGrouping = false;
    radGridView1.MasterTemplate.MultiSelect = true;

- Start the application and drag the filter button without releasing the mouse button.

Workaround:
class MyGridFilterRowBehavior : GridFilterRowBehavior
{
    protected override bool ProcessMouseSelection(Point mousePosition, GridCellElement currentCell)
    {
        if (currentCell is GridFilterCellElement)
        {
            return false;
        }
        return base.ProcessMouseSelection(mousePosition, currentCell);
    }
}

BaseGridBehavior gridBehavior = radGridView1.GridBehavior as BaseGridBehavior;
gridBehavior.UnregisterBehavior(typeof(GridViewFilteringRowInfo));
gridBehavior.RegisterBehavior(typeof(GridViewFilteringRowInfo), new MyGridFilterRowBehavior());

To reproduce:
- Crete a self-reference hiearchy and show the grid lines.
- Start the application expand some rows scroll to the bottom and then scroll to the right.
- The issue is easily reproducible with the example from the documentation.

Workaround:
class MyDataCellElement : GridDataCellElement
{
    public MyDataCellElement(GridViewColumn col, GridRowElement row) : base(col, row)
    {
    }

    protected override SizeF ArrangeOverride(SizeF finalSize)
    {
        SelfReferenceCellLayout selfReferenceLayout = this.SelfReferenceLayout;
        if (selfReferenceLayout != null)
        {
            GridDataRowElement dataRowElement = this.RowElement as GridDataRowElement;

            if (dataRowElement != null && this.ColumnInfo.OwnerTemplate != null)
            {
                dataRowElement.SelfReferenceLayout.StackLayoutElement.Visibility = ElementVisibility.Visible;
            }
        }
        else
        {
            GridDataRowElement dataRowElement = this.RowElement as GridDataRowElement;

            if (dataRowElement != null && this.ColumnInfo.OwnerTemplate != null)
            {
                dataRowElement.SelfReferenceLayout.StackLayoutElement.Visibility = ElementVisibility.Collapsed;
            }
        }
        return base.ArrangeOverride(finalSize);
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(GridDataCellElement);
        }
    }
}

How to reproduce:

Create a grid in self referencing hierarchy and set up a filter expression like this: 
 this.radGridView1.FilterDescriptors.Expression = "Name = \"bin\" AND (FileSystemInfoType = \"Folder\" OR Id = \"2\")"
Immediately after the expression is set, the OR logical operator is substituted with AND

To reproduce:
- Show a message box in the CellValidating event handler.
- Start the application, edit a cell and directly press the vertical scroll bottom arrow.

 

To reproduce:
1 - Add a grid view in a form.
2 - Add 14 - 17 records in grid view.
3 - Allow alternating color for grid view.
4 - Do sorting by clicking on header column (click 3 times).
5 - Alternating color display wrong position.

Workaround:
void radGridView1_SortChanged(object sender, GridViewCollectionChangedEventArgs e)
{
    foreach (GridRowElement row in this.radGridView1.TableElement.ViewElement.ScrollableRows.Children)
    {
        row.Synchronize();
    } 
}

To reproduce:
public RadForm1()
{
    InitializeComponent();
    Controls.Add(new RadGridView
    {
        Dock = DockStyle.Fill,
        ShowHeaderCellButtons = true,
        ReadOnly = true,
        AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill,
        EnableFiltering = true,
        DataSource = Enumerable.Range(0, 24000) //Increase this number when you can't reproduce this issue
        .Select(x => new { Name = string.Format("Item {0}", x) }),
    });
}

Workaround:
class MyListFillter : RadListFilterPopup
{
    public MyListFillter(GridViewDataColumn column, bool groupDateValues) : base(column, groupDateValues)
    {
    }

    protected override void OnButtonOkClick(EventArgs e)
    {
        FilterOperator filterOperator = FilterOperator.IsEqualTo;
        var listFilterElement = typeof(RadListFilterPopup).GetField("listFilterElement", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(this) as FilterMenuTreeElement;

        switch (listFilterElement.SelectedMode)
        {
            case ListFilterSelectedMode.All:
                filterOperator = FilterOperator.None;
                break;
            case ListFilterSelectedMode.Null:
                filterOperator = FilterOperator.IsNull;
                break;
            case ListFilterSelectedMode.NotNull:
                filterOperator = FilterOperator.IsNotNull;
                break;
        }

        if (filterOperator != FilterOperator.IsEqualTo)
        {
            SetFilterOperator(filterOperator);
            this.ClosePopup(RadPopupCloseReason.CloseCalled);
        }
        else
        {
            CompositeFilterDescriptor compositeFilterDescriptor = new CompositeFilterDescriptor();
            compositeFilterDescriptor.PropertyName = base.DataColumn.Name;
            compositeFilterDescriptor.LogicalOperator = FilterLogicalOperator.Or;
           
            if (listFilterElement.SelectedValues.Count > this.GetDistinctValuesTable().Count / 2)
            {                    
                foreach (DictionaryEntry entry in this.GetDistinctValuesTable())
                {
                    if (!listFilterElement.SelectedValues.Contains(entry.Key))
                    {
                        foreach (object value in (ArrayList)entry.Value)
                        {
                            FilterDescriptor descriptor;
                            if (base.DataColumn is GridViewDateTimeColumn || base.DataColumn.DataType == typeof(DateTime) ||
                                base.DataColumn.DataType == typeof(DateTime?))
                            {
                                descriptor = new DateFilterDescriptor(base.DataColumn.Name, FilterOperator.IsNotEqualTo, (DateTime?)value, false);
                            }
                            else
                            {
                                descriptor = new FilterDescriptor(base.DataColumn.Name, FilterOperator.IsNotEqualTo, value);
                            }
                            compositeFilterDescriptor.FilterDescriptors.Add(descriptor);
                        }
                    }
                }
            }
            else
            {
                foreach (DictionaryEntry entry in listFilterElement.SelectedValues)
                {
                    foreach (object value in (ArrayList)entry.Value)
                    {
                        FilterDescriptor descriptor;
                        if (base.DataColumn is GridViewDateTimeColumn || base.DataColumn.DataType == typeof(DateTime) ||
                            base.DataColumn.DataType == typeof(DateTime?))
                        {
                            descriptor = new DateFilterDescriptor(base.DataColumn.Name, FilterOperator.IsEqualTo, (DateTime?)value, false);
                        }
                        else
                        {
                            descriptor = new FilterDescriptor(base.DataColumn.Name, FilterOperator.IsEqualTo, value);
                        }
                        compositeFilterDescriptor.FilterDescriptors.Add(descriptor);
                    }
                }
            }
            base.FilterDescriptor = compositeFilterDescriptor;
            OnFilterConfirmed();
        }
    }
}

To reproduce:
- Add logo in the header and export the document.

Workaround:
void exporter_HeaderExported(object sender, ExportEventArgs e)
{
    PdfEditor ed = e.Editor as PdfEditor;
    using (MemoryStream ms = new MemoryStream())
    {
        System.Drawing.Image.FromFile(@"C:\img\delete.png").Save(ms, System.Drawing.Imaging.ImageFormat.Png);
        ed.TranslatePosition(e.Rectangle.Width - 30, e.Rectangle.Y);
        ed.DrawImage(ms, new System.Windows.Size(30,30));
    }
}

To reproduce: use the following code snippet and click the button twice.

private DataTable GetTable01()
{
    DataTable table = new DataTable();
    table.Columns.Add("Dosage", typeof(int));
    table.Columns.Add("Drug", typeof(string));
    table.Columns.Add("Patient", typeof(string));
    table.Columns.Add("Date", typeof(DateTime));

    // Here we add five DataRows.
    table.Rows.Add(25, "Indocin", "David", DateTime.Now);
    table.Rows.Add(50, "Enebrel", "Sam", DateTime.Now);
    table.Rows.Add(10, "Hydralazine", "Christoff", DateTime.Now);
    table.Rows.Add(21, "Combivent", "Janet", DateTime.Now);
    table.Rows.Add(100, "Dilantin", "Melanie", DateTime.Now);

    return table;
}

private void button1_Click_1(object sender, EventArgs e)
{
    DataTable table01 = GetTable01();

    radGridView1.Rows.Clear(); 

    radGridView1.DataSource = table01;
}

WORKAROUND I:
this.radGridView1.DataError += radGridView1_DataError;

private void radGridView1_DataError(object sender, GridViewDataErrorEventArgs e)
{
    if (e.Exception is ArgumentException && e.Exception.Message == "Cannot clear this list.")
    {
        this.radGridView1.BeginUpdate();

        while (this.radGridView1.Rows.Count > 0)
        {
            this.radGridView1.Rows.RemoveAt(0);
        }

        this.radGridView1.EndUpdate();
    }
}

WORKAROUND II:
 while (this.radGridView1.Rows.Count > 0)
{               
     this.radGridView1.Rows.RemoveAt(this.radGridView1.Rows.Count - 1);
}

To reproduce: use the following code snippet and try to filter the second column:

public Form1()
{
    InitializeComponent();

    this.radGridView1.Columns.Add("Id");
    GridViewDateTimeColumn dateTimeColumn = new GridViewDateTimeColumn();
    dateTimeColumn.FilteringMode = GridViewTimeFilteringMode.Date;
    dateTimeColumn.Name = "NullDateTimeColumn";
    dateTimeColumn.HeaderText = "Order date";
    dateTimeColumn.FieldName = "OrderDate"; 
    dateTimeColumn.FormatString = "{0:dd/MM/yyyy}";
    this.radGridView1.Columns.Add(dateTimeColumn);

    dateTimeColumn = new GridViewDateTimeColumn();
    dateTimeColumn.FilteringMode = GridViewTimeFilteringMode.Date;
    dateTimeColumn.Name = "DateTimeColumn";
    dateTimeColumn.HeaderText = "Not Null Date date";
    dateTimeColumn.FieldName = "NotNullDate";
    dateTimeColumn.FormatString = "{0:dd/MM/yyyy}";
    this.radGridView1.Columns.Add(dateTimeColumn);
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    for (int i = 0; i < 50; i++)
    {
        if (i % 2 == 0)
        {
            this.radGridView1.Rows.Add(new object[] { i, null, DateTime.Today });
        }
        else
        { 
            this.radGridView1.Rows.Add(new object[] { i, DateTime.Now.AddDays(i), DateTime.Today });
        }
    }
    this.radGridView1.EnableFiltering = true;
    this.radGridView1.MasterTemplate.ShowHeaderCellButtons = true;
    this.radGridView1.MasterTemplate.ShowFilteringRow = false;
}

private void radGridView1_FilterPopupRequired(object sender, FilterPopupRequiredEventArgs e)
{
    e.FilterPopup = new RadListFilterPopup(e.Column, true);
}

Workaround: use a specific DateTime value instead of null (e.g. 1/1/1800) and use the CellFormatting event to clear the displayed text. Afterwards, in the RadListFilterPopup it is necessary to hide the text for the null value date nodes:

public Form1()
{
    InitializeComponent();

    this.radGridView1.Columns.Add("Id");
    GridViewDateTimeColumn dateTimeColumn = new GridViewDateTimeColumn();
    dateTimeColumn.FilteringMode = GridViewTimeFilteringMode.Date;
    dateTimeColumn.Name = "NullDateTimeColumn";
    dateTimeColumn.HeaderText = "Order date";
    dateTimeColumn.FieldName = "OrderDate"; 
    dateTimeColumn.FormatString = "{0:dd/MM/yyyy}";
   
    this.radGridView1.Columns.Add(dateTimeColumn);

    dateTimeColumn = new GridViewDateTimeColumn();
    dateTimeColumn.FilteringMode = GridViewTimeFilteringMode.Date;
    dateTimeColumn.Name = "DateTimeColumn";
    dateTimeColumn.HeaderText = "Not Null Date date";
    dateTimeColumn.FieldName = "NotNullDate";
    dateTimeColumn.FormatString = "{0:dd/MM/yyyy}";
    this.radGridView1.Columns.Add(dateTimeColumn);
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    for (int i = 0; i < 50; i++)
    {
        if (i % 2 == 0)
        {
            this.radGridView1.Rows.Add(new object[] { i, new DateTime(1800, 1, 1), DateTime.Today });
        }
        else
        { 
            this.radGridView1.Rows.Add(new object[] { i, DateTime.Now.AddDays(i), DateTime.Today });
        }
    }
    this.radGridView1.EnableFiltering = true;
    this.radGridView1.MasterTemplate.ShowHeaderCellButtons = true;
    this.radGridView1.MasterTemplate.ShowFilteringRow = false;
}

private void radGridView1_FilterPopupRequired(object sender, FilterPopupRequiredEventArgs e)
{
    RadListFilterPopup popup = new RadListFilterPopup(e.Column, true);
    e.FilterPopup = popup;
    popup.PopupOpened += popup_PopupOpened;
}

private void popup_PopupOpened(object sender, EventArgs args)
{
    RadListFilterPopup popup = sender as RadListFilterPopup;
    if (popup != null)
    {
        popup.MenuTreeElement.TreeView.NodeFormatting += TreeView_NodeFormatting;
        popup.MenuTreeElement.TreeView.Focus();
    }
}

private void TreeView_NodeFormatting(object sender, TreeNodeFormattingEventArgs e)
{
    if (e.NodeElement.ContentElement.Text.Contains("1800"))
    {
        e.NodeElement.ContentElement.DrawText = false;
        e.NodeElement.ExpanderElement.Opacity = 0;
        e.NodeElement.ExpanderElement.ShouldHandleMouseInput = false;
        e.NodeElement.ShouldHandleMouseInput = false;
    }
    else
    { 
        e.NodeElement.ContentElement.DrawText = true;
        e.NodeElement.ExpanderElement.Opacity = 1;
        e.NodeElement.ExpanderElement.ShouldHandleMouseInput = true;
        e.NodeElement.ShouldHandleMouseInput = true;
    }
}

private void radGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
{
    GridViewDateTimeColumn dtCol = e.Column as GridViewDateTimeColumn;
    if (e.Row is GridViewDataRowInfo && dtCol != null && ((DateTime)e.CellElement.Value).Year == 1800)
    {
        e.CellElement.Text = string.Empty;
    }
}

Workaround
this.radGridView1.Templates["MyEmptyTemplate"]..AddNewRowPosition = SystemRowPosition.Top

Then handle the ViewCellFormatting event and get notifications when the hierarchy tabs are being clicked.
private void radGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
	GridDetailViewCellElement detailCell = e.CellElement as GridDetailViewCellElement;
	if (detailCell != null) {
		detailCell.PageViewElement.ItemSelected -= PageViewElement_ItemSelected;
		detailCell.PageViewElement.ItemSelected += PageViewElement_ItemSelected;
	}
}

private void PageViewElement_ItemSelected(object sender, RadPageViewItemSelectedEventArgs e)
{
	if (e.SelectedItem.Text == "MyEmptyTemplate") {
		this.SetsGridViewTemplate.AddNewRowPosition = SystemRowPosition.Bottom;
	}
}