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.