[FIX] it is possible that temporary NSManagedObjects get created with a ...

[FIX] it is possible that temporary NSManagedObjects get created with a nil context, in which case the desiredConcurrencyIdentifier may be nil, even though correct concurrency is being respected.  This seems to fix the numerous logged errors.  Example case:

// create a temporary NSManagedObject
NSEntityDescription *entity = [NSEntityDescription entityForName:@"EntityName"
                                              inManagedObjectContext:moc];
__block NSManagedObject *obj = nil;
 [moc performBlockAndWait:^{
        obj = [[NSManagedObject alloc] initWithEntity:entity insertIntoManagedObjectContext:nil];
        obj.key = @"value";
    }];

// use obj here, validate etc. in the correct moc performBlock

// insert into some moc at some later point / or not
 [moc performBlockAndWait:^{
    [moc insertObject:obj];
    }];