I've recently upgraded SQL prompt to version 5.1 (build 184.108.40.206).
Beforehand I'd disabled SQL Prompt in BIDS and everything worked fine.
Since the upgrade, every time I load BIDS the checkbox is ticked (even if I unticked it then restarted BIDS properly) and previews still crash BIDS regardless.
Worked out the problem. I didn't have admin rights so when I tried to update Visual Studio to remove the add-in, the settings weren't saved.
Ran VS as admin and made the changes, now it all works fine.