I recently removed my development branch using git branch -D development in order to clear some commits it had that I did not want to preserve between feature commits.
The result was that finishing the (still present and active) feature branch I had open through the Git Flow button caused a complaint that git flow had not yet been initialized (it had).
This resulted in the rather annoying need to track down its real location within the SourceTree.app/Contents/ hierarchy and run git flow init again by hand.
Ideally Source Tree should automatically attempt to recover from detectable and straightforward git flow init issues such as this, saving people the lengthy run-around.