To reproduce:
Search a specific text by focusing the search box programmatically and the using the SendKeys method:
private void radButton1_Click(object sender, EventArgs e)
{
    GridSearchCellElement searchCell = radGridView1.TableElement.GetCellElement(radGridView1.MasterView.TableSearchRow, null) as GridSearchCellElement;
    if (searchCell != null)
    {
        searchCell.SearchTextBox.Focus();
        searchCell.SearchTextBox.Text = string.Empty;
        SendKeys.Send("t");
        SendKeys.Send("e");
        SendKeys.Send("s");
        SendKeys.Send("t");
    }
}

Workaround: 
Repeat the search in the SearchProgressChanged event:

radGridView1.MasterView.TableSearchRow.SearchProgressChanged += TextationSearchProgressHandler;

protected void TextationSearchProgressHandler(object sender, SearchProgressChangedEventArgs e)
{
    if (e.SearchFinished && null != radGridView1.TableElement)
    {
        GridSearchCellElement searchCell = radGridView1.TableElement.GetCellElement(radGridView1.MasterView.TableSearchRow, null) as GridSearchCellElement;
        if (searchCell != null
             && searchCell.SearchTextBox.TextBoxItem.Text != e.SearchCriteria)
        {
            radGridView1.MasterView.TableSearchRow.Search(searchCell.SearchTextBox.TextBoxItem.Text);
        }
    }
}

To reproduce:

public class Item
{
    public int Id { get; set; }

    public string Name { get; set; }

    public DateTime Date { get; set; }

    public Item(int id, string name, DateTime date)
    {
        this.Id = id;
        this.Name = name;
        this.Date = date;
    }
}
public Form1()
{
    InitializeComponent();
    List<Item> items = new List<Item>();
    for (int i = 0; i < 10; i++)
    {
        items.Add(new Item(i,"Item" + i,DateTime.Now.AddHours(i)));
    }
    this.radGridView1.DataSource = items;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
}

private void Form1_Load(object sender, EventArgs e)
{
    System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("en-GB");
    this.radGridView1.Columns["Date"].ExcelExportFormatString = "M/d/yyyy h:mm tt";
    this.radGridView1.Columns["Date"].ExcelExportType = DisplayFormatType.Custom;

    GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
    spreadExporter.ExportVisualSettings = true;
    SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
    string fileName = @"..\..\exportedFile" + DateTime.Now.ToLongTimeString().Replace(":", "_") + ".xlsx";
    spreadExporter.RunExport(fileName, exportRenderer);
    Process.Start(fileName);
}

Workaround: use a custom BaseGridBehavior

public Form1()
{
    InitializeComponent();

    this.radGridView1.GridBehavior = new MyBaseGridBehavior();
}

public class MyBaseGridBehavior : BaseGridBehavior
{
    public override bool ProcessKey(KeyEventArgs keys)
    {
        if (keys.Control && this.GridControl.CurrentColumn == null)
        {
            return false;
        }

        return base.ProcessKey(keys);
    }
}

To reproduce:
- Add some rows to a grid.
- Sort the rows.
- Delete a row.
- The current row is not the next row.

Workaround:
Dim t As Test = RadGridView1.CurrentRow.DataBoundItem
Dim index As Integer = Me.RadGridView1.ChildRows.IndexOf(Me.RadGridView1.CurrentRow)
datasource.Remove(t)
Me.RadGridView1.CurrentRow = Me.RadGridView1.ChildRows(index)

To reproduce:

1. Run the attached application. 

2. Click into the cell in Column One and expand the combo box

3. While the combo box is still expanded, right click on any header cell in the grid

Step 3 should result in a NullReferenceException

Workaround:
Friend Class MyGridHeaderCellElement
    Inherits GridHeaderCellElement
 
    Public Sub New(ByVal col As GridViewColumn, ByVal row As GridRowElement)
        MyBase.New(col, row)
    End Sub
    Protected Overrides Sub ShowContextMenu()
        If Me.ViewTemplate IsNot Nothing Then
            MyBase.ShowContextMenu()
        End If
    End Sub
    Protected Overrides ReadOnly Property ThemeEffectiveType() As Type
        Get
            Return GetType(GridHeaderCellElement)
        End Get
    End Property
End Class

Private Sub radGridView1_CreateCell(ByVal sender As Object, ByVal e As GridViewCreateCellEventArgs)
    If Object.ReferenceEquals(e.CellType, GetType(GridHeaderCellElement)) Then
        e.CellType = GetType(MyGridHeaderCellElement)
    End If
 
End Sub

To reproduce:

public Form1() 
{
    InitializeComponent();

    DataTable dt = new DataTable();
    for (int i = 0; i < 5; i++)
    {
        dt.Columns.Add("Col"+i);
    }
    for (int i = 0; i < 2000; i++)
    {
        DataRow row = dt.NewRow();
        foreach (DataColumn col in dt.Columns)
        {
            row[col.ColumnName] = "Data" + i + "." + dt.Columns.IndexOf(col);
        }
        dt.Rows.Add(row);
    }
    this.radGridView1.DataSource = dt;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
}

private void radButton1_Click(object sender, EventArgs e)
{
    RadPrintPreviewDialog dialog = new RadPrintPreviewDialog();
    dialog.Document = this.radPrintDocument1;
    dialog.ShowDialog();
}

Workaround: this.radGridView1.AutoSizeRows = true;

To reproduce:

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.RadGridView1.AutoGenerateColumns = False

    Dim column = New GridViewTextBoxColumn
    column.Name = "Name"
    column.FieldName = "ChildItem"
    column.DataType = GetType(CustomItem)
    column.DataTypeConverter = New CustomItemTypeConverter()

    Me.RadGridView1.Columns.Add(column)
    Me.RadGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill

    Dim objects =
        {
            New CustomContainer With {.ChildItem = New CustomItem With {.Name = "A"}},
            New CustomContainer With {.ChildItem = New CustomItem With {.Name = "B"}},
            New CustomContainer With {.ChildItem = New CustomItem With {.Name = "C"}}
        }

    Me.RadGridView1.DataSource = objects
End Sub

Private Sub RadGridView_EditorRequired(sender As Object, e As EditorRequiredEventArgs) Handles RadGridView1.EditorRequired
    e.EditorType = GetType(GridSpinEditor)
End Sub

Class CustomContainer
    Public Property ChildItem As CustomItem
End Class

Class CustomItem
    Public Property Name As String
End Class

Class CustomItemTypeConverter
Inherits TypeConverter

    Public Overrides Function CanConvertFrom(context As ITypeDescriptorContext, sourceType As Type) As Boolean
        If sourceType = GetType(Decimal) Then
            Return True
        End If

        Return MyBase.CanConvertFrom(context, sourceType)
    End Function

    Public Overrides Function CanConvertTo(context As ITypeDescriptorContext, destinationType As Type) As Boolean
        If destinationType = GetType(Decimal) OrElse destinationType = GetType(String) OrElse destinationType = GetType(CustomItem) Then
            Return True
        End If

        Return MyBase.CanConvertTo(context, destinationType)
    End Function

    Public Overrides Function ConvertTo(context As ITypeDescriptorContext, culture As System.Globalization.CultureInfo, _
                                        value As Object, destinationType As Type) As Object
        If TypeOf value Is CustomItem Then
            Dim customValue = DirectCast(value, CustomItem)

            If destinationType = GetType(Decimal) Then
                Return Microsoft.VisualBasic.AscW(customValue.Name.Chars(0))
            ElseIf destinationType = GetType(String) Then
                Return customValue.Name
            ElseIf destinationType = GetType(CustomItem) Then
                Return customValue
            Else
                Return MyBase.ConvertTo(context, culture, value, destinationType)
            End If
        End If

        Return MyBase.ConvertTo(context, culture, value, destinationType)
    End Function

    Public Overrides Function ConvertFrom(context As ITypeDescriptorContext, culture As System.Globalization.CultureInfo, value As Object) As Object
        If TypeOf value Is Decimal Then
            Dim decValue = DirectCast(value, Decimal)
            Dim intValue = CInt(decValue)
            Dim charValue = Microsoft.VisualBasic.ChrW(intValue)

            Return New CustomItem With {.Name = New String(charValue, 1)}
        End If

        Return MyBase.ConvertFrom(context, culture, value)
    End Function

End Class


Workaround:

Public Class MyEditor
    Inherits GridSpinEditor

    Public Overrides ReadOnly Property IsModified() As Boolean
        Get
            If Me.originalValue Is Nothing Then
                Return (Me.Value IsNot Nothing AndAlso Me.Value <> DBNull.Value)
            End If

            If Me.Value Is Nothing Then
                Return (Me.originalValue IsNot Nothing AndAlso Me.originalValue <> DBNull.Value)
            End If
            Dim column As GridViewDataColumn = (DirectCast(Me.OwnerElement, GridDataCellElement)).ColumnInfo
            If column.DataTypeConverter IsNot Nothing AndAlso column.DataTypeConverter.CanConvertTo(Me.OwnerElement, GetType(Decimal)) Then
                Return Not column.DataTypeConverter.ConvertTo(Me.OwnerElement, column.FormatInfo, Me.originalValue, GetType(Decimal)).Equals(Convert.ToDecimal(Me.Value))
            End If

            Return Not Convert.ToDecimal(Me.originalValue).Equals(Convert.ToDecimal(Me.Value))
        End Get
    End Property
End Class


Private Sub RadGridView_EditorRequired(sender As Object, e As EditorRequiredEventArgs) Handles RadGridView1.EditorRequired
    e.EditorType = GetType(MyEditor)
End Sub

All event handlers should be made virtual.
All controls should be public or have properties.

To reproduce:

private void Form2_Load(object sender, EventArgs e) 
{ 
    this.productsTableAdapter.Fill(this.nwindDataSet.Products); 
    this.categoriesTableAdapter.Fill(this.nwindDataSet.Categories);

    this.radGridView1.AutoGenerateHierarchy = true;
    this.radGridView1.DataSource = this.nwindDataSet;
    this.radGridView1.DataMember = "Categories";
    this.radGridView1.MasterTemplate.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    this.radGridView1.MasterTemplate.Templates.First().AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

    GridViewSummaryItem summaryItem = new GridViewSummaryItem();
    summaryItem.Name = "CategoryID";
    summaryItem.Aggregate = GridAggregateFunction.Count;
    GridViewSummaryRowItem summaryRowItem = new GridViewSummaryRowItem();
    summaryRowItem.Add(summaryItem);
    this.radGridView1.SummaryRowsTop.Add(summaryRowItem);

    GridViewSummaryItem summaryItem2 = new GridViewSummaryItem();
    summaryItem2.Name = "UnitPrice";
    summaryItem2.Aggregate = GridAggregateFunction.Sum;
    GridViewSummaryRowItem summaryRowItem2 = new GridViewSummaryRowItem();
    summaryRowItem2.Add(summaryItem2);
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop.Add(summaryRowItem2); 
}

private void SetNumberFormat()
{
    this.radGridView1.Columns["CategoryID"].FormatString = "{0:F4}";
    this.radGridView1.SummaryRowsTop[0][0].FormatString = "{0:F4}";

    this.radGridView1.MasterTemplate.Templates.First().Columns["UnitPrice"].FormatString = "{0:F4}";
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop[0][0].FormatString = "{0:F4}";
}

private void radButton1_Click(object sender, EventArgs e)
{
    SetNumberFormat();
}

Workaround: Clear and add back the summary rows when a value in the child template is changed. 
private void radButton1_Click(object sender, EventArgs e)
{
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop.Clear();

    GridViewSummaryItem summaryItem2 = new GridViewSummaryItem();
    summaryItem2.Name = "UnitPrice";
    summaryItem2.Aggregate = GridAggregateFunction.Sum;
    GridViewSummaryRowItem summaryRowItem2 = new GridViewSummaryRowItem();
    summaryRowItem2.Add(summaryItem2);
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop.Add(summaryRowItem2); 

    SetNumberFormat();
}

Workaround: use custom editor element

private void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(RadMultiColumnComboBoxElement))
    {
        e.EditorType = typeof(MyRadMultiColumnComboBoxElement);
    }
}

public class MyRadMultiColumnComboBoxElement : RadMultiColumnComboBoxElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadMultiColumnComboBoxElement);
        }
    }

    protected override void ProcessKeyDown(object sender, KeyEventArgs e)
    {
        base.ProcessKeyDown(sender, e);

        FieldInfo fi = this.GetType().BaseType.BaseType.GetField("oldTextValue", BindingFlags.Instance | BindingFlags.NonPublic);
        fi.SetValue(this, this.textBox.Text.Substring(0, this.textBox.SelectionStart));
    }
}

To reproduce:

1. Add a RadGridView with a GridViewMultiComboBoxColumn. Enable the auto filter functionality for this column and add an appropriate FilterDescriptor to the RadMultiColumnComboBoxElement.
2. Using the keyboard arrows only (no mouse), navigate to the GridViewMultiComboBoxColumn.
3. Type "ba" by using the keyboard. The "b" is lost and only the "a" gets to the filter. I expect the filter to show "ba".

Workaround: use custom row behavior

public Form1()
{
    InitializeComponent();
    //register the custom row  behavior
    BaseGridBehavior gridBehavior = radGridView1.GridBehavior as BaseGridBehavior;
    gridBehavior.UnregisterBehavior(typeof(GridViewDataRowInfo));
    gridBehavior.RegisterBehavior(typeof(GridViewDataRowInfo), new CustomGridDataRowBehavior());
}

public class CustomGridDataRowBehavior : GridDataRowBehavior
{
    protected override bool ProcessAlphaNumericKey(KeyPressEventArgs keys)
    {
        bool result = base.ProcessAlphaNumericKey(keys);
        if (this.IsInEditMode &&
            (this.BeginEditMode == RadGridViewBeginEditMode.BeginEditOnKeystroke ||
             this.BeginEditMode == RadGridViewBeginEditMode.BeginEditOnKeystrokeOrF2))
        {
            if (this.GridViewElement.ActiveEditor is RadMultiColumnComboBoxElement)
            {
                this.GridViewElement.ActiveEditor.Value = keys.KeyChar;
                if (this.GridViewElement.IsInEditMode)
                {
                    RadMultiColumnComboBoxElement mccb = (RadMultiColumnComboBoxElement)this.GridViewElement.ActiveEditor;
                    RadTextBoxItem textBoxItem = mccb.TextBoxElement.TextBoxItem;
                    textBoxItem.Clear();
                    textBoxItem.Text =  keys.KeyChar.ToString();
                    textBoxItem.SelectionStart = 1;
                    textBoxItem.SelectionLength = 0;
                }

                return true;
            }
        }
        return result;
    }
}

To reproduce:
public abstract class Base
{
    public abstract string Name { get; }
}

public class Child : Base
{
    public override string Name
    {
        get { return "Child"; }
    }
}

public class Child2 : Base
{
    public override string Name
    {
        get { return "Child2"; }
    }
}

public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();

        var list = new List<Base>();
        list.Add(new Child());
        list.Add(new Child());
        list.Add(new Child2());

        radGridView1.DataSource = list.ToArray();
    }
}

Workaround:
 radGridView1.DataSource = list;

Workaround: skip the summary rows by creating custom GridViewSearchRowInfo:  

private void radGridView1_CreateRowInfo(object sender, GridViewCreateRowInfoEventArgs e)
{
    if (e.RowInfo is GridViewSearchRowInfo)
    {
        e.RowInfo = new CustomGridViewSearchRowInfo(e.ViewInfo);
    }
}


public class CustomGridViewSearchRowInfo : GridViewSearchRowInfo
        {
            public CustomGridViewSearchRowInfo(GridViewInfo viewInfo) : base(viewInfo)
            {
            }

            protected override bool MatchesSearchCriteria(string searchCriteria, GridViewRowInfo row, GridViewColumn col)
            {
                if (row is GridViewSummaryRowInfo)
                {
                    return false;
                }
                return base.MatchesSearchCriteria(searchCriteria, row, col);
            }
        }

To reproduce:
radGridView1.GridNavigator.SelectFirstRow();
for (int i = 0; i < radGridView1.RowCount; i++)
{
    var result = radGridView1.GridNavigator.SelectNextRow(1);
}

Workaround:
- Move to the last row first:
radGridView1.GridNavigator.SelectLastRow();

I've attached a sample project.  Here are the steps to repeat this issue:
1. Run the application
2. Click column A and drag it to the right to reorder it, scrolling the window as it goes
3. You are eventually presented with a NullReferenceException
You may have to try this several times with different columns before the problem manifests.  I have found that moving just beyond the right edge of the main form and moving the mouse cursor up and down a little helps.  I appears this problem only occurs in grids with many columns, so that fast scrolling can occur for multiple seconds when reordering columns.

Workaround:

Sub New()
    InitializeComponent()

    Me.RadGridView1.GridViewElement.RegisterService(New MyRadGridViewDragDropService(Me.RadGridView1.GridViewElement))

End Sub

Public Class MyRadGridViewDragDropService
Inherits RadGridViewDragDropService
    Public Sub New(gridViewElement As RadGridViewElement)
        MyBase.New(gridViewElement)

    End Sub

    Public Overrides ReadOnly Property Name As String
        Get
            Return "RadGridViewDragDropService"
        End Get
    End Property
    Protected Overrides Sub PrepareDragHint(dropTarget As Telerik.WinControls.ISupportDrop)
        Dim dragDropBehavior As IGridDragDropBehavior = Me.GetDragDropBehavior()
        If dragDropBehavior Is Nothing OrElse dragDropBehavior.DragHint Is Nothing _
            OrElse dragDropBehavior.DragHint.Image Is Nothing Then
            Return
        End If

        Dim dropTargetItem As RadItem = TryCast(dropTarget, RadItem) 
        If dropTargetItem IsNot Nothing AndAlso dropTargetItem.ElementTree IsNot Nothing _
            AndAlso Not dropTargetItem.ElementTree.Control.Equals(Me.GridViewElement.ElementTree.Control) Then
            Return
        End If
         
        dragDropBehavior.UpdateDropContext(TryCast(Me.Context, ISupportDrag), dropTarget, Me.beginPoint)
        Dim hintSize As Size = dragDropBehavior.GetDragHintSize(dropTarget)
        Dim image As New Bitmap(hintSize.Width, hintSize.Height)
        Dim temp As Graphics = Graphics.FromImage(image)
        dragDropBehavior.DragHint.Paint(temp, New RectangleF(PointF.Empty, hintSize))
        temp.Dispose()

        Dim dragHintWindow As New RadLayeredWindow()
        GetType(RadGridViewDragDropService).GetField("dragHintWindow", _
         Reflection.BindingFlags.NonPublic Or Reflection.BindingFlags.Instance).SetValue(Me, dragHintWindow)
        dragHintWindow.BackgroundImage = image
    End Sub
End Class

To reproduce: use the following code snippet. From the filtering box, when you select "Null" and then select "All", the following error occurs:
Item has already been added. Key in dictionary: '(Blanks)'  Key being added: '(Blanks)'

DataTable dt = new DataTable();
dt.Columns.Add("Id", typeof(int));
dt.Columns.Add("Name", typeof(string));

dt.Rows.Add(1,"A");
dt.Rows.Add(2, "");
dt.Rows.Add(3, null);
dt.Rows.Add(4, "B");
dt.Rows.Add(5, "C");
dt.Rows.Add(6, "");
this.radGridView1.DataSource = dt;
this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
this.radGridView1.EnableFiltering = true;
this.radGridView1.ShowFilteringRow = false;
this.radGridView1.ShowHeaderCellButtons = true;

Workaround:  this.radGridView1.FilterPopupInitialized += radGridView1_FilterPopupInitialized;

RadListFilterDistinctValuesTable selectedValues;

private void radGridView1_FilterPopupInitialized(object sender, Telerik.WinControls.UI.FilterPopupInitializedEventArgs e)
{
    RadListFilterPopup popup = e.FilterPopup as RadListFilterPopup;
    selectedValues = popup.MenuTreeElement.SelectedValues;
    popup.MenuTreeElement.TreeView.NodeCheckedChanged += TreeView_NodeCheckedChanged;
}

private void TreeView_NodeCheckedChanged(object sender, TreeNodeCheckedEventArgs e)
{
    if (e.Node.CheckState == Telerik.WinControls.Enumerations.ToggleState.Off)
    {
        if (selectedValues.Contains(e.Node.Text))
        {
            selectedValues.Remove(e.Node.Text);
        }
    }
}

To reproduce:
- Add an expression column with nullable type as a data source. 
- Open the property builder.

Workaround: 
Set the data type at run time. 

To reproduce:
- Add some columns to the grid, make sure that most of the columns are not visible (the user must scroll to view them).
- Drag and drop the second column at the last position in the grid.
- The result can be seen on the attached image. 

Workaround:

Private Sub Columns_CollectionChanged(ByVal sender As Object, ByVal e As Telerik.WinControls.Data.NotifyCollectionChangedEventArgs)
      Me.RadGridView1.TableElement.ViewElement.UpdateRowsWhenColumnsChanged()
End Sub

To reproduce:

public Form1()
{
    InitializeComponent();
    BindingList<Item> items = new BindingList<Item>();
    
    for (int i = 0; i < 30; i++)
    {
        if (i % 3 == 0)
        {
            items.Add(new Item(i,"Item" + i, IndeterminateBoolean.YesAndNo));
        }
        else if (i % 3 == 1)
        {
            items.Add(new Item(i, "Item" + i, IndeterminateBoolean.Yes));
        }
        else
        {
            items.Add(new Item(i, "Item" + i, IndeterminateBoolean.No));
        }
    }
    this.radGridView1.DataSource = items;
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    this.radGridView1.EnableFiltering = true; 
    this.radGridView1.ShowHeaderCellButtons = true;
    this.radGridView1.ShowFilteringRow = false; 
}

public class Item
{
    public int Id { get; set; }

    public string Name { get; set; }

    public IndeterminateBoolean IsActive { get; set; }

    public Item(int id, string name, IndeterminateBoolean isActive)
    {
        this.Id = id;
        this.Name = name;
        this.IsActive = isActive;
    }
}

public enum IndeterminateBoolean
{
    No,
    Yes,
    YesAndNo
}

Workaround: use custom filtering http://docs.telerik.com/devtools/winforms/gridview/filtering/custom-filtering

To reproduce:

public Form1()
{
    InitializeComponent();
    BindingList<Item> items = new BindingList<Item>();
    
    for (int i = 0; i < 30; i++)
    {
        if (i % 3 == 0)
        {
            items.Add(new Item(i,"Item" + i, IndeterminateBoolean.YesAndNo));
        }
        else if (i % 3 == 1)
        {
            items.Add(new Item(i, "Item" + i, IndeterminateBoolean.Yes));
        }
        else
        {
            items.Add(new Item(i, "Item" + i, IndeterminateBoolean.No));
        }
    }
    this.radGridView1.DataSource = items;
    GridViewCheckBoxColumn checkBox = new GridViewCheckBoxColumn("CheckBoxCol");
    checkBox.DataTypeConverter = new IndeterminateBooleanToggleStateConverter();
    checkBox.FieldName = "IsActive";
    checkBox.ThreeState = true;
    this.radGridView1.Columns.Add(checkBox);
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    this.radGridView1.EnableFiltering = true;

    this.radGridView1.ShowHeaderCellButtons = true;
    this.radGridView1.ShowFilteringRow = false;
}

public class Item
{
    public int Id { get; set; }

    public string Name { get; set; }

    public IndeterminateBoolean IsActive { get; set; }

    public Item(int id, string name, IndeterminateBoolean isActive)
    {
        this.Id = id;
        this.Name = name;
        this.IsActive = isActive;
    }
}

public enum IndeterminateBoolean
{
    No,
    Yes,
    YesAndNo
}

public class IndeterminateBooleanToggleStateConverter : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(ToggleState);
    }
    
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(ToggleState);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        if (value is ToggleState)
        {
            switch ((ToggleState)value)
            {
                case ToggleState.On:
                    return IndeterminateBoolean.Yes;
                case ToggleState.Off:
                    return IndeterminateBoolean.No;
                case ToggleState.Indeterminate:
                default:
                    return IndeterminateBoolean.YesAndNo;
            }
        }
        else
        {
            return base.ConvertFrom(context, culture, value);
        }
    }
    
    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(ToggleState))
        {
            if (value is IndeterminateBoolean)
            {
                switch ((IndeterminateBoolean)value)
                {
                    case IndeterminateBoolean.Yes:
                        return ToggleState.On;
                    case IndeterminateBoolean.No:
                        return ToggleState.Off;
                    case IndeterminateBoolean.YesAndNo:
                    default:
                        return ToggleState.Indeterminate;
                }
            }
            if (value is string)
            {
                switch (((string)value ?? string.Empty).Trim())
                {
                    case "Yes":
                        return ToggleState.On;
                    case "No":
                    case "":
                        return ToggleState.Off;
                    case "Both":
                    default:
                        return ToggleState.Indeterminate;
                }
            }
            else
            {
                return base.ConvertTo(context, culture, value, destinationType);
            }
        }
        else
        {
            return base.ConvertTo(context, culture, value, destinationType);
        }
    }
}

Workaround: use the custom filtering functionality: http://docs.telerik.com/devtools/winforms/gridview/filtering/custom-filtering

You can access the RadGridView.FilterDescriptors collection and according to the FilterDescriptor.Expression property to determine whether the row will be visible or not.