When I try to register the TreeNodesCollection of an UltraTree as a CAB-UIExtensionSite like this

RootWorkItem.UIExtensionSites.RegisterSite(UIExtensionSiteNames.WindowTemplatesTreeNodes, this.Shell.WindowTemplateNodes);

I get the following error message:

"No UIElementAdapterFactory register for type Infragistics.Win.UltraWinTree.TreeNodesCollection."

I checked with your documentation and it says

UltraTreeNode
The following objects can be registered as extension sites:

• Nodes collection
• WinTree™ control
• individual UltraTreeNode element

Do I have to register a special AdapterFactory as stated in the error message or what else might I do wrong?

Thanks in advance and best regards, Gerald