I upgraded to SourceTree 1.9.5 today, but it started to crash on every launch. I happened to have an old copy of 1.5.7 so I removed the new version and installed the old version in its place. I was then able to open repositories. I tried switching back to 1.9.5 (because maybe it was just corrupted user data or something) but then I was able to open the bookmark browser but not open any of the repositories. Every time I double-clicked on a repo it would try to open its window and crash.
I've attached the crash report. (sourcetree-crashlog-2014-07-08.txt)