static MainWindow()
{
    EventManager.RegisterClassHandler(typeof(RadMenuItem), PreviewMouseLeftButtonUpEvent, new MouseButtonEventHandler(OnRadMenuItemPreviewMouseLeftButtonUp), true);
}

private static void OnRadMenuItemPreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    RadMenuItem radMenuItem = (RadMenuItem)sender;

    RadContextMenu radContextMenu = radMenuItem.ParentOfType<RadContextMenu>();

    if (radContextMenu != null)
    {
        PropertyInfo menuPropertyInfo = radMenuItem.GetType().GetProperty("Menu", BindingFlags.Public | BindingFlags.Instance);
        if (menuPropertyInfo != null)
        {
            MethodInfo setMethod = menuPropertyInfo.GetSetMethod(true);
            if (setMethod != null)
            {
                Application.Current.Dispatcher.BeginInvoke(new Action(() =>
                {
                    setMethod.Invoke(radMenuItem, new object[] { radContextMenu });
                }), System.Windows.Threading.DispatcherPriority.Background);
            }
        }
    }
}

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)