SourceTree crashed while I wasn't actively using it. I was in Xcode, and may just have saved a file modifying its Git status.

The interesting bits of the crash report (full log is attached). Note there is no SourceTree code in the stack...

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x000070000ad61628Thread 0:: Dispatch queue: com.apple.main-thread
0 libsystem_platform.dylib 0x00007fff645f1f49 _platform_memmove$VARIANT$Haswell + 41
1 com.apple.CoreFoundation 0x00007fff3ce06c47 -[__NSArrayI getObjects:range:] + 183
2 com.apple.Foundation 0x00007fff3f00c029 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 293
3 com.apple.Foundation 0x00007fff3f00bc0b -[NSObject(NSKeyValueObservingPrivate) _notifyObserversOfChangeFromValuesForKeys:toValuesForKeys:] + 699
4 com.apple.CoreFoundation 0x00007fff3cf4393b forEachObserver + 219
5 com.apple.CoreFoundation 0x00007fff3cf43b78 -[CFPrefsSource _notifyObserversOfChangeFromValuesForKeys:toValuesForKeys:] + 72
6 com.apple.CoreFoundation 0x00007fff3cf86b52 __78-[CFPrefsSearchListSource deferredNotifyOfChangesFromDictionary:toDictionary:]_block_invoke + 34
7 com.apple.CoreFoundation 0x00007fff3cfaa422 __44-[_CFXPreferences copyObservationConnection]_block_invoke_3 + 50
8 com.apple.CoreFoundation 0x00007fff3ce6952c _CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK_ + 12...