Created Unassigned: 3.0.0 AvalonDock MVVM binding defect [22448]

Since version 3.0.0, the MVVM-Binding of AvalonDock seems to be defective.

When AvalonDock is bound (__AnchorablesSource__) to an __ObservableCollection__, changing this collection (Clearing it, specifically) raises a __NullPointerException__.

Example-Project attached.

Steps to reproduce (in the Example-Project):
- Start project
- Directly press button at the bottom of the window. (do not move / click the panels)

This works flawlessly with 2.9.0