Problems with ResizeSlot() and nested SplitContainers

<docking:DockSite x:Name="dockSite" >
 <docking:SplitContainer Orientation="Horizontal">
   <docking:ToolWindowContainer x:Name="CommandContainer">
     <docking:ToolWindow x:Name="CommandToolWindow" Title="">
     </docking:ToolWindow>
   </docking:ToolWindowContainer>
 <docking:SplitContainer Orientation="Horizontal">
 <docking:ToolWindowContainer x:Name="MainContainer">
   <docking:ToolWindow x:Name="DebugOutContainer" Title="" >
      <docking:DocumentWindow x:Name="DebugOutput" Title="Status Window"
                              Description="Status and racindow"                
                              FileName="debug.rtf" IsReadOnly="True">
        <TextBox Name="DebugOutputText" BorderThickness="0" TextWrapping="NoWrap"
             Text="" 
             IsReadOnly="{Binding ElementName=debugOutput,
                         Path=IsReadOnly}"/>
      </docking:DocumentWindow>
    </docking:ToolWindow>
   </docking:ToolWindowContainer>
  </docking:SplitContainer>
 </docking:SplitContainer>   
</docking:DockSite>

ResultControl res = new ResultControl(_dynamicSet, _mainThreadDispatcher,_debugFunction);
ToolWindow window = new ToolWindow(dockSite, "", "", new BitmapImage( 
                                     new Uri("/Resources/lock.png",UriKind.Relative)),  
                                   res );
window.Dock(MainContainer, Direction.Top);
SplitContainer= MainVertSplit(SplitContainer)VisualTreeHelperExtended.GetAncestor(MainContainer, typeof(SplitContainer));
if (null != MainVertSplit)
{
  MainVertSplit.ResizeSlots(2, 1);
}
SplitContainer MainHorzSplit = (SplitContainer)VisualTreeHelperExtended.GetAncestor(CommandContainer, typeof(SplitContainer));
if (null != MainHorzSplit)
{
  MainHorzSplit.ResizeSlots(1, 2);
}