Steps to reproduce:

1. Download this sample project: https://www.codeproject.com/KB/WPF/483507/Version_03_Edi_RecentFilesTW.zip
2. Update AvalonDock version to 3.5.12.
3. Drag 'Recent Files' anchorable window and drop it to the document's pane.
4. Drag 'File Stats' anchorable window and drop it to the document's pane.
5. Close 'File Stats' with X.
6. Close 'Recent Files' with X.
7. Choose 'Tools' from the menu and click 'RecentFiles'.
8. It throws:
   System.InvalidCastException: 'Unable to cast object of type 'Xceed.Wpf.AvalonDock.Layout.LayoutAnchorable' to type 'Xceed.Wpf.AvalonDock.Layout.ILayoutPanelElement'.'

Other exception:

1. Repeat step 1-6.
2. Choose 'Tools' from the menu and click 'Properties'.
3. Repeat step 2.
4. It throws:
   System.ArgumentOutOfRangeException: 'Index must be within the bounds of the List. Parameter name: index'