MLKit: save is not that save
[MLLibrary save] tries to retain objects gone with the wind or with pending CoreData faults:
3 Foundation 0x30e7e588 -[NSObject(NSKeyValueObservingPrivate) _willChangeValuesForKeys:] + 196
4 CoreData 0x302ac760 -[NSFaultHandler turnObject:intoFaultWithContext:] + 184
5 CoreData 0x302cd3f0 -[NSManagedObjectContext(_NSInternalAdditions) _didSaveChanges] + 2640
6 CoreData 0x302aef16 -[NSManagedObjectContext save:] + 1182
7 VLC for iOS 0x0067cb52 -[MLMediaLibrary save] (MLMediaLibrary.m:196)
8 VLC for iOS 0x0067a576 -[MLCrashPreventer willParseFile:] (MLCrashPreventer.m:96)
9 VLC for iOS 0x006807b8 -[ThumbnailOperation fetchThumbnail] (MLThumbnailerQueue.m:62)