"Active Files" popup of TabbedMdiHost contains empty items only

Hello Actipro,

my TabbedMdiContainer will be filled in code-behind with DocumentWindows. The menu "active files" will automatically be filled with items but only the image (ImageSource property) will be displayed. The filenames are missing here.

<docking:Workspace>
	<docking:TabbedMdiHost>
		<docking:TabbedMdiContainer Background="{DynamicResource {x:Static themes:AssetResourceKeys.WindowBackgroundNormalBrushKey}}" />
	</docking:TabbedMdiHost>
</docking:Workspace>

<docking:DocumentWindow x:Class="MyApp.SaveDocumentWindow"
                        ...
                        xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking" 
                        FileName="{Binding FileNameBase, Mode=OneTime}"
                        Description="{Binding FileNameBase, Mode=OneTime}"
                        Header="{Binding FileNameBase, Mode=OneTime}"
                        ImageSource="{StaticResource FileImageKey">
    <!-- Content -->
</docking:DocumentWindow>

private DocumentWindow CreateDocumentWindow(SingleChordsViewModel singleChords, bool isAlreadyOpened)
{
	DocumentWindow documentWindow;

	if (isAlreadyOpened)
	{
		documentWindow = _dockSite.DocumentWindows.Single(dw => dw.DataContext.Equals(singleChords));
	}
	else
	{
		documentWindow = new SaveDocumentWindow(_dockSite) { DataContext = singleChords };
	}
	
	// activate the DocumentWindow
	documentWindow.Activate();

	return documentWindow;
}

Have I missed anything? Or is there a workaround to fill/modify the popup items manually?

Regards
Sascha