RadTreeView needs a collection for its ItemsSource property in order to function correctly and that is why the drag and drop stops working when you set the ItemsSource to be your ListCollectionView. What you can do in your case though, is use the default view of your collection. You can do so as follows:
sourceTree.ItemsSource = testCollection;
ListCollectionView view = (ListCollectionView)CollectionViewSource.GetDefaultView(sourceTree.ItemsSource);
Let me know if this solves your problem.
All the best,
the Telerik team
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