I was having the same issue. It started shortly after adding the ToDo list to the dock panel.
To fix:
Make a backup copy of the dst file.
Edit the dst file and location the DockSite0 section.
Find the entry for TabDockClients and remove any clients that you may have added recently. I believe the default value for that entry is, "RefactoringForm,PatchForm,FindReferencsForm"
As soon as I removed "Todo List" the exception stopped appearing.