Today we run into a problem with FileMaker 12.
On every (!) launch here, we get a crash report dialog with a lot of details including this message:
*** Terminating app due to uncaught exception 'NSRangeException', reason: 'Cannot remove an observer
for the key path "areInspectorsVisible" from because it is not registered as an observer.'
This could be a bug in FileMaker or one of the Apple frameworks. Probably due to running FileMaker 12 on macOS 10.12 Sierra. The removeObserver method is called where it shouldn't be.
Now I can use my MBS Plugin to fix this with a work around: The new plugin watches for calls to removeObserver method and puts the call in an try/catch block. If the exception occurs, we can log it and continue. FileMaker doesn't see the exception and does not terminate. If no exception happens, the call just continues normally and nobody will notice.
If you run into the same problem, let me know and I can maybe help you with the same issue with an updated MBS Plugin.