Reason:

This happens when the object’s context does not belong to the current thread. One of many ways that mistake can manifest itself.

Debug:

When the crash occurs do the following:

1. Check the thread in which the crash occured (Xcode > Debug Navigator)
2. On the console print, the concurrency type of the NSManagedObjectContext
3. If the concurrency type is mainQueueConcurrencyType then it needs to be on the main thread otherwise on the background threads.

Console command to check concurrency type:

p context.concurrencyType

In the sample command above the NSManagedObjectContext was stored in the variable context

Console Output:

(NSManagedObjectContextConcurrencyType) $R4 = mainQueueConcurrencyType