var dropDownLists = radRichTextBox.Document.EnumerateChildrenOfType<SdtRangeStart>()
     .Where(x => x.SdtProperties is ComboBoxProperties)
     .Select(x => x.SdtProperties).OfType<ComboBoxProperties>();
 var defaultItemString = LocalizationManager.GetString("Documents_ContentControlsGenerator_ListItem");
 foreach (var item in dropDownLists)
 {
     if (item.Items.Count > 0 && item.Items.Any(x => x.Value != defaultItemString) && item.Items.Any(x => x.Value == defaultItemString))
     {
         var occurrence = item.Items.FirstOrDefault(x => x.Value == defaultItemString);
         while (occurrence != null)
         {
             item.Items.Remove(occurrence);
             occurrence = item.Items.FirstOrDefault(x => x.Value == defaultItemString);
         }
     }
 }

static MainWindow()
{
    EventManager.RegisterClassHandler(typeof(RadMenuItem), RadMenuItem.LoadedEvent, new RoutedEventHandler(OnMenuItemLoaded));
}

private static void OnMenuItemLoaded(object sender, RoutedEventArgs e)
{
    var menuItem = (RadMenuItem)sender;
    if (menuItem.DataContext is DefinedName context)
    {
        menuItem.Header = new TextBlock() { Text = context.Name };
    }
}