MCPE sometimes crashes when opening certain worlds

MCPE sometimes crashes during the "Building terrain" stage when opening some worlds that we've had for a while. It seems to have the same backtrace – I'll attach a crash report that you should be able to symbolicate.

Not sure if there's a way to extract a saved world from an iPad backup (maybe via the backup extractor at supercrazyawesome.com?), but if you had directions I could attach that too..