private void RadSyntaxEditor_PreviewKeyDown(object sender, System.Windows.Input.KeyEventArgs e)
 {
     if (KeyboardModifiers.IsControlDown && e.Key == Key.Space)
     {		
        e.Handled = true;

		var editor = (RadSyntaxEditor)sender;
        CaretPosition startPosition = new CaretPosition(editor.CaretPosition);
        CaretPosition endPosition = new CaretPosition(editor.CaretPosition);
        editor.IntelliPrompts.CompletionListWindow.Show(startPosition, endPosition);
		editor.IntelliPrompts.CompletionListWindow.HorizontalOffset = 0;
		editor.IntelliPrompts.CompletionListWindow.VerticalOffset = 0;
     }
 }

 private void RadSyntaxEditor_Loaded(object sender, System.Windows.RoutedEventArgs e)
 {
    var editor = (RadSyntaxEditor)sender;
    var caret = editor.FindChildByType<Telerik.Windows.Controls.SyntaxEditor.UI.Caret>();
    editor.IntelliPrompts.CompletionListWindow.PlacementTarget = caret;
    editor.IntelliPrompts.CompletionListWindow.Placement = System.Windows.Controls.Primitives.PlacementMode.Bottom;
 }

System.ArgumentException: 'Requested distance is outside the content of the associated document.'PresentationFramework.dll!System.Windows.Documents.TextPointer.TextPointer(System.Windows.Documents.TextContainer textContainer, int offset, System.Windows.Documents.LogicalDirection direction)
  PresentationFramework.dll!System.Windows.Documents.TextContainer.CreatePointerAtOffset(int offset, System.Windows.Documents.LogicalDirection direction) 
  PresentationFramework.dll!System.Windows.Documents.TextParentUndoUnit.Do() 
  PresentationFramework.dll!MS.Internal.Documents.UndoManager.Redo(int count) 
  PresentationFramework.dll!System.Windows.Documents.TextStore.RedoQuietly(int count)  PresentationFramework.dll!System.Windows.Documents.TextStore.SetFinalDocumentState(MS.Internal.Documents.UndoManager undoManager, System.Collections.Stack imeChangeStack, int appChangeCount, int imeSelectionAnchorOffset, int imeSelectionMovingOffset, int appSelectionAnchorOffset, int appSelectionMovingOffset)  PresentationFramework.dll!System.Windows.Documents.TextStore.HandleCompositionEvents(int previousUndoCount)  PresentationFramework.dll!System.Windows.Documents.TextStore.GrantLockWorker(MS.Win32.UnsafeNativeMethods.LockFlags flags)   PresentationFramework.dll!System.Windows.Documents.TextStore.RequestLock(MS.Win32.UnsafeNativeMethods.LockFlags flags, out int hrSession)

dialog.Filter = "Special files|.*_FileNamePart.txt"

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.dateRangePicker.Loaded += DateRangePicker_Loaded;
    }

    private void DateRangePicker_Loaded(object sender, RoutedEventArgs e)
    {
        DateRangeMaskedInput dateRangeMaskedInput = this.dateRangePicker.FindChildByType<DateRangeMaskedInput>();

        if (dateRangeMaskedInput != null)
        {
            dateRangeMaskedInput.ValueChanging += DateRangeMaskedInput_ValueChanging;
        }
    }

    private void DateRangeMaskedInput_ValueChanging(object? sender, Telerik.Windows.Controls.MaskedInput.RadMaskedInputValueChangingEventArgs e)
    {
        if (e.NewValue.ToString().Any(char.IsLetter))
        {
            e.Handled = true;
        }
    }
}