Actually, you will not be able to work with the Items collection directly while RadGridView is in use. You may try something like follows:
> gridViewSource = this.playersGrid.ItemsSource as IList<
and remove/add items from/to the gridViewSouce.
Furthermore, I tested the issue you reported - the fact that the ItemsSource of RadTreeView is null and I was not able to reproduce it. Please see the sample project attached and let me know whether you can get the same behavior on it.
the Telerik team
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