NSFetchedResultsController attempting to insert nil object

From an Apple Technical Support Engineer:

To protect the integrity of the datastore, Core Data catches some exceptions that happen during its operations. Sometimes this means that if Core Data calls your code through a delegate method, Core Data may end up catching exceptions your code threw.

Multi-threading errors are the most common cause of mysterious Core Data issues.

In this case, Core Data caught an exception through your controllerDidChangeContent: method, caused by trying to use insertObject:atIndex.

The most likely fix is to ensure that all your NSManagedObject code is encapsulated inside performBlock: or performBlockAndWait: calls.

In iOS 8 and OSX Yosemite, Core Data gains the ability to detect and report violations of its concurrency model. It works by throwing an exception whenever your app accesses a managed object context or managed object from the wrong dispatch queue. You enable the assertions by passing -com.apple.CoreData.ConcurrencyDebug 1 to your app on the command line via Xcodeʼs Scheme Editor.

Ole Begemann has a great writeup of the new feature.