Workrarund:
If possible create tool-windows with added user controls
public static ContentUserControl CreateContentUserControl(RadDock dock)
{
    ContentUserControl uc = new ContentUserControl();
    if (dock != null)
    {
        ToolWindow hostWindow = new ToolWindow("ContentUserControl");
        hostWindow.Controls.Add(uc);
        hostWindow.Name = uc.Name;
        dock.FloatWindow(hostWindow);
    }

    return uc;
}