It is possible that you are trying to get the ScrollViewer before it has been created. That is why you get a null reference. You can try getting it in some of the events that fire after the TreeList components have been created i.e. the RowLoadedEvent. Mind however that Synchronous scrolling in two or more TreeLists can be tricky i.e. if one of the Lists has more expanded nodes than the other. You will have to think of a way to handle such scenarios.
Hope this helps!
All the best,
the Telerik team
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