Created Unassigned: [Version 2.0] LayoutAnchorable and 'menuSite Null' [16354]

We encounter an problem to add WPF menu on the content of LayoutAnchorable within a LayoutAnchorablePaneGroup.

The problem can be easily reproduced as
https://avalondock.codeplex.com/discussions/429063

Click on "Tool Window 2" tab will cause an exception.

And the exception message is exactly the same as
https://avalondock.codeplex.com/discussions/447154

Thanks