There are large number of items for the TreeList (for example 3000) and you have all them initially collapsed. For example you could modify the IsExpandedBinding demo to have 3000 items. Then scroll down so the row with Drinks and Count = 2194 is at the top of the control and expand it. The result is that both that row and the row with Drinks and Count = 2199 expand.
TreeListView is trying to expand its item which is empty.
As a workaround you can try executing the selection in a Dispatcher with DispatcherPriority.Render. This may not apply in the case when there are many nodes outside the View area.
The goal is to expand a second node each time you expand a particular node from the RadTreeListView. The data of the second node is loaded on demand, but it is not shown when the node itself is expanded.