I can suggest you the following approaches:
1) Since you keep a dictionary of the Panes in the ViewModel, you can add some logic in the View, which will tell the ViewModel that a Pane has been closed and remove it from the dictionary.
2) You can bind to the IsHidden
property of the Pane.
3) There is a third approach which is illustrated in this help resource:
However, I haven't try it with RadDocking control, so I cannot confirm that is working smoothly, but it is a good practice and it's worth the try.
Hope this information was helpful.
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