I've had to delete Skitch from all of my computers because this is so annoying. I refuse to be forced into syncing these applications.
Same problem here (screen shot attached). Running Safari version 10.1 (12603.1.30.0.34) on Sierra 10.12.4. Deleting and reinstalling the extension fixed the issue, and I logged a ticket with support. Posting here just so there's another point towards a development resolution.