To reproduce:
            string fileName = @"..\..\..\exported" + DateTime.Now.Hour + DateTime.Now.Minute + DateTime.Now.Second + ".xlsx";
            SpreadExport spreadExporter = new SpreadExport(radGridView1);
            spreadExporter.ExportVisualSettings = false;      
            spreadExporter.RunExport(fileName);


Workaround:
spreadExporter.WorkbookCreated += spreadExporter_WorkbookCreated;
private void spreadExporter_WorkbookCreated(object sender, WorkbookCreatedEventArgs e)
{

    Telerik.Windows.Documents.Spreadsheet.PropertySystem.CellStyle defaultStyle = e.Workbook.Styles.Add("DefaultStyle");
    defaultStyle.FontSize = Telerik.Windows.Documents.Spreadsheet.Utilities.UnitHelper.PointToDip(11);
    Telerik.Windows.Documents.Spreadsheet.Model.Worksheet sheet = e.Workbook.Worksheets.First();
    sheet.Cells[0, 0, sheet.UsedCellRange.RowCount, sheet.UsedCellRange.ColumnCount].SetStyleName("DefaultStyle");
    sheet.Columns[sheet.UsedCellRange].AutoFitWidth();
}

To reproduce use the following snippets in the CellFormatting event of the exporter:
 ThemableColor red = new ThemableColor(System.Windows.Media.Colors.Red);
 CellBorders redBorders = new CellBorders(new CellBorder(CellBorderStyle.Medium, red));            

 e.CellStyleInfo.Borders = redBorders;

or 

ThemableColor red = new ThemableColor(System.Windows.Media.Colors.Red);
CellBorders fourBorders = new CellBorders();

fourBorders.Top = new CellBorder(CellBorderStyle.Thin, red);
fourBorders.Bottom = new CellBorder(CellBorderStyle.Thin, red);
fourBorders.Right = new CellBorder(CellBorderStyle.Thin, red);
fourBorders.Left = new CellBorder(CellBorderStyle.Thin, red);

e.CellStyleInfo.Borders = fourBorders;

Hello,

I succeed to reproduce the GridViewComboboxColumn exception in this forum post:
http://www.telerik.com/forums/nullreferenceexception-4a6181b2453b#cwDrbIqzp0CPxcgh90b4rQ

I attach a sample project, the database (SQL Server 2012 Express) and a video from the exception.

To reproduce:
- Run the project,
- Sort the column "Állapot" descending.
- Click on column and drop down the list.
- Choose an another value, and click very fast twice. On a slow PC is much easier to reproduce the issue. The important thing, that you need select a value from the combobox and select another row very fast. (See the attached video)

I use the latest Trial version of Winforms.

If you have any question, please contact me.

Best Regards,
László

Workaround:

Private Sub gridMunkak_CreateCell(sender As Object, e As GridViewCreateCellEventArgs) Handles gridMunkak.CreateCell
    If e.CellType = GetType(GridComboBoxCellElement) Then
        e.CellElement = New MyGridComboBoxCellElement(e.Column, e.Row)
    End If
End Sub

Public Class MyGridComboBoxCellElement
    Inherits GridComboBoxCellElement
    Public Sub New(column As GridViewColumn, row As GridRowElement)
        MyBase.New(column, row)
    End Sub

    Public Overrides Sub SetContent()
        If Me.ColumnInfo IsNot Nothing Then
            MyBase.SetContent()
        End If
    End Sub
    Protected Overrides ReadOnly Property ThemeEffectiveType() As Type
        Get
            Return GetType(GridComboBoxCellElement)
        End Get
    End Property
End Class

To reproduce:
1. Add a RadGridView and a RadButton.
2. Populate the grid with data and call the BestFitColumns( BestFitColumnMode.AllCells) method (or resize the columns).
3. Set its RightToLeft property to Windows.Forms.RightToLeft.Yes.
3. In the RadButton.Click event handler call the RadGridView.PrintPreview(). As a result the columns are shrunk. Please see the attached gif file.

Workaround:

Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
    Me.RadGridView1.BeginUpdate()
    Me.RadGridView1.PrintPreview()
    Me.RadGridView1.EndUpdate()
    Me.RadGridView1.BestFitColumns(BestFitColumnMode.AllCells)
End Sub

To reproduce:
- Create a custom column that uses RadTextBoxControlElement as permanent editor.
- Add the column to the grid. 
- Scroll the grid so the custom column is not visible.
- Put the the grid in edit mode and change the current cell.  

Workaround:
- Disable the IME support:
public class MyRadTextBoxControlElement : RadTextBoxControlElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTextBoxControlElement);
        }
    }
    protected override void OnLoaded()
    {
        this.InvalidateMeasure();
    }
}

If a RadGridView instance is not disposed explicitly and it is left for the finalizer to dispose, a null reference exception will be thrown. This can happen if the grid is not in the control tree of a form. The exception is caused by the fact that the SelfReferenceDataProvider is disposed twice in this case.

Workaround: 

        RadImageShape hint;

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            InitializeComponent();

            hint = radGridView1.TableElement.RowDragHint;
            new Windows7Theme();
            radGridView1.ThemeName = "Windows7";

            radGridView1.TableElement.RowDragHint = hint;
          }

The issue appears when selecting multiple cells using mouse drag outside of the bounds of the control. 
Add a large number of columns (100) and a few rows, press the first cell and drag quickly as far to the right as possible. 
When you scroll back and check the selection, you will see that some cells are not selected.

ADD. RadGridView - add ability to move the column with the expanders to different position then the first one

Resolution: 
Set the SelfReferenceExpanderColumn property of the MasterTemplate to the preferred column. Here is the code snippet: 
radGridView1.MasterTemplate.SelfReferenceExpanderColumn = this.radGridView1.MasterTemplate.Columns[4];

To reproduce:
- Open the Search Row sample in the demo application.
- Type some text in the search textbox.
- Using the context menu of the search textbox add some unicode control characters.

private void radGridView1_CellEditorInitialized(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
{
    GridColorPickerEditor colorEditor = e.ActiveEditor as GridColorPickerEditor;
    if (colorEditor!=null)
    {
        GridColorPickerElement colorPicker = colorEditor.EditorElement  as GridColorPickerElement;
        colorPicker.ReadOnly = true;
    }
}

Please refer to the attached gif file.

Workaround:
private void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(GridColorPickerEditor))
    {
        e.Editor = new GridColorPickerEditor();
    }
}

To reproduce:
- Add grid to a form and populate it with data.
- Show and hide the the excel like filtering several times.

To reproduce:
- Add GridViewCheckBoxColumn and set the EnableHeaderCheckBox property to true.
- Mark all check boxes and change the data source of the grid (use one where not all values are set to true).

Workaround:
Add new column when the data source is changed.

To reproduce:
- Add checkbox column to a grid and enable filtering.
- Filter on other column so there are no rows visible.
- The header cell checkbox is checked automatically.

Workaround:
void radGridView_CreateCell(object sender, GridViewCreateCellEventArgs e)
{
    if (e.CellType == typeof(GridCheckBoxHeaderCellElement))
    {
        e.CellElement = new GridCheckBoxHeaderCellElement(e.Column,e.Row);

        ((GridCheckBoxHeaderCellElement)e.CellElement).CheckBox.ToggleStateChanging += CheckBox_ToggleStateChanging;
    }
}

void CheckBox_ToggleStateChanging(object sender, StateChangingEventArgs args)
{
    if (radDevices.ChildRows.Count == 0)
    {
        args.Cancel = true;
    }
}

To reproduce: 
- Bind the grid to a self reference data, it should contain nullable bool value as well.
- Add checkbox column:
GridViewCheckBoxColumn chkBoxColumn = new GridViewCheckBoxColumn();
chkBoxColumn.EnableHeaderCheckBox = true;
chkBoxColumn.ThreeState = true;
chkBoxColumn.EditMode = EditMode.OnValueChange;

- Start and uncheck and check one of the cells (in a data row)

Workaround:
public class MyGridCheckBoxHeaderCellElement : GridCheckBoxHeaderCellElement
{
    public MyGridCheckBoxHeaderCellElement(GridViewColumn column, GridRowElement row) : base(column,row)
    {
    }

    protected override bool SetCheckBoxState()
    {
        bool hasNullValue = false;
        foreach (GridViewRowInfo row in this.ViewInfo.Rows)
        {
            object cellValue = row.Cells[this.ColumnIndex].Value;
            if (cellValue == null)
            {
                hasNullValue = true;
            }
        }

        if (!hasNullValue)
        {
            return base.SetCheckBoxState();
        }

        SetCheckBoxState(ToggleState.Indeterminate);
        return false;
    }
}

To reproduce:
- Enable the search row in the 50000 rows example.
- Set the 5 last rows RowIndex value to null.
- Set the filter of the RowIndex column to "Is null" and then to "Is not null"
- Type 499 in the search row.
- Set the filter of the RowIndex column to "Is null" and then to "Is not null" again.

Workaround:
public class MyGridViewSearchRowInfo : GridViewSearchRowInfo
{
    public MyGridViewSearchRowInfo(GridViewInfo viewInfo) : base(viewInfo)
    {
    }

    protected override int GetCurrentCellTraverserColumnIndex()
    {
        if (this.ViewTemplate.MasterTemplate.Owner.CurrentRow == null)
        {
            return -1;
        }
        
        return base.GetCurrentCellTraverserColumnIndex();
    }
}
//change the row like this:
void radGridViewDemo_CreateRowInfo(object sender, GridViewCreateRowInfoEventArgs e)
{
    if (e.RowInfo is GridViewSearchRowInfo)
    {
        e.RowInfo = new MyGridViewSearchRowInfo(e.ViewInfo);
    }
}

To repriduce:
- Add a grid with Office2007Theme applied and sort by one of the columns
- Set RightToLeft to true
- Make sure the column width is no wider than the text

Workaround:
        void radGridView1_ViewCellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e) 
        { 
            GridHeaderCellElement headerCell = e.CellElement as GridHeaderCellElement;   

            if (headerCell != null) 
            { 
                if (radGridView1.ThemeName == "Office2007Silver" && e.Column.IsSorted) 
                { 
                    headerCell.Arrow.Alignment = ContentAlignment.TopCenter; 
                    headerCell.Arrow.Margin = new Padding(0, 1, 0, 0); 
                } 
            } 
        }

To reproduce:
1. Add text box and check box column to the grid. Add a filter descriptor to the text box column
2. Bind it to a DataTable with some data
3. Clear the columns
4. Add the columns once again => the exception will be thrown 

Workaround.

1. Create the following cell element:
    class MyHeaderCell : GridCheckBoxHeaderCellElement
    {
        public MyHeaderCell(GridViewColumn column, GridRowElement row)
            : base(column, row)
        {

        }

        protected override bool SetCheckBoxState()
        {
            if (this.ColumnIndex == -1)
            {
                return false;
            }

            return base.SetCheckBoxState();
        }
    }
2. Subscribe to the grid's CreateCell event
3. Put the modified cell in action:
        void radGridView_CreateCell(object sender, GridViewCreateCellEventArgs e)
        {
            if (e.CellType == typeof(GridCheckBoxHeaderCellElement))
            {
                e.CellType = typeof(MyHeaderCell);
            }
        }

To reproduce:
- Add combobox column and filter the grid so just one row is visible.
- In CellValueChanged event show a message box.
- Change the value in the combo box column and click in the white space of the grid.

Workaround:
public class MyGridComboBoxCellElement : GridComboBoxCellElement
{
    public MyGridComboBoxCellElement(GridViewColumn column, GridRowElement row) : base(column, row)
    {
    }

    public override void SetContent()
    {
        if (this.ColumnInfo != null)
        {
            base.SetContent();
        }
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(GridComboBoxCellElement);
        }
    }
}

void radGridView1_CreateCell(object sender, GridViewCreateCellEventArgs e)
{
    if (e.CellType == typeof(GridComboBoxCellElement))
    {
        e.CellElement = new MyGridComboBoxCellElement(e.Column, e.Row);
    }
}

Add RadPageView with couple pages
Add a grid to one of the pages
Start editing a cell
Click another page item (tab) to change the selected page
=> even though no validation is performed, the selected page is not changed unless a second click is performed. The first click just closes the editor and the second one changes the selected page.