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As of version 2.6.1, SourceTree crashes often…usually when I'm not doing anything in the app (and sometimes when I'm not even using my Mac).
I can provide crash logs if it helps.
I'm tracking a deep issue related to this and theming for the next update.
Brian GanningerSenior Mac Developer, Sourcetree
I just reset my preferences by closing SourceTree and deleting "~/Library/Preferences/com.torusknot.SourceTreeNotMAS.plist". This seems to have fixed a really annoying problem with SourceTree's working tree list and diff viewer flickering and resetting their scroll position every 10 seconds. Maybe it'll fix the crashing issue too.
I'll report back if I see another crash. If not, you can assume this fixed the problem.