SourceTree shows the same error dialog lots of times simultaneously when it gets confused by the repository index

I was playing with the git-new-workdir script and trying to get it running on Windows, and managed to screw up my index a bit. That's completely my fault and I'm trying to recover it using git fsck (failing that I'll just copy it from another computer that is fairly up to date).

However, SourceTree panicked quite badly when when it tried to show its log/branch view. See the attached screenshot.

This re-occurs every time I open it, so I cannot actually use any of my repositories at the moment since the dialogs re-appear faster than I can dismiss them.