title: crash on coredata ios8
body: <p>I have a crash on CoreData when I save : </p>
  
  <pre><code>2014-09-16 09:51:58.273 My_app[2678:105246] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString appendString:]: nil argument'
  *** First throw call stack:
  (
      0   CoreFoundation                      0x00000001087413f5 __exceptionPreprocess + 165
      1   libobjc.A.dylib                     0x00000001083dabb7 objc_exception_throw + 45
      2   CoreFoundation                      0x000000010874132d +[NSException raise:format:] + 205
      3   CoreFoundation                      0x000000010871274f mutateError + 159
      4   CoreData                            0x000000010672ae56 -[_NSSQLGenerator prepareMasterReorderStatementPart2ForRelationship:] + 118
      5   CoreData                            0x0000000106792fd8 -[NSSQLAdapter newCorrelationMasterReorderStatementPart2ForRelationship:] + 72
      6   CoreData                            0x00000001067a9751 -[NSSQLiteConnection writeCorrelationMasterReordersFromTracker:] + 817
      7   CoreData                            0x00000001067aa061 -[NSSQLiteConnection writeCorrelationChangesFromTracker:] + 65
      8   CoreData                            0x000000010679c617 -[NSSQLCore writeChanges] + 1351
      9   CoreData                            0x00000001066dfadf -[NSSQLCore saveChanges:] + 479
      10  CoreData                            0x00000001066b0ee4 -[NSSQLCore executeRequest:withContext:error:] + 484
      11  CoreData                            0x00000001067868f2 __65-[NSPersistentStoreCoordinator executeRequest:withContext:error:]_block_invoke + 4354
      12  CoreData                            0x000000010678e7ee gutsOfBlockToNSPersistentStoreCoordinatorPerform + 190
      13  libdispatch.dylib                   0x00000001091e27f4 _dispatch_client_callout + 8
      14  libdispatch.dylib                   0x00000001091c9848 _dispatch_barrier_sync_f_invoke + 365
      15  CoreData                            0x00000001067813d5 _perform + 197
      16  CoreData                            0x00000001066b0ac8 -[NSPersistentStoreCoordinator executeRequest:withContext:error:] + 504
      17  CoreData                            0x00000001066d9d2d -[NSManagedObjectContext save:] + 1213
  </code></pre>
  
  <p>The same code works in ios7.</p>
  
  <p>Is there someone who has the same crash ? </p>