I had one file pending, which I wanted to stop tracking, so I right clicked and chose to Stop Tracking, accepting the changes. This file then changes its status to a delete (no entry sign).

Then to commit that change I right clicked on it again, this time choosing "Stop Tracking", which fails and I get an "Oops!" dialogue, and SourceTree closes.

The commit does succeed if I check the checkbox, and use the commit button on the File Status tab.