The current (4.1.2) version of Sourcetree crashes constantly on my machine. The crash reports all look identical, so this should be trivial to fix. The cause of the crash is:
Crashing on exception: *** -[__NSArrayM objectAtIndexedSubscript:]: index 0 beyond bounds for empty array
You will find a sample crash report [at this link|http://hairer.org/Sourcetree.crash].