title: Swift CoreData: error: Serious application error. Exception was caught during Core Data change processing.
body: <p>I am writing one program on iOS and very race I am facing this error: </p>
  
  <blockquote>
    <p>2015-11-06 10:57:24.289 NETFNET[2503:976392] CoreData: error: Serious application error.  Exception was caught during Core Data change processing.  This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification.  -[__NSCFSet addObject:]: attempt to insert nil with userInfo (null)
    2015-11-06 10:57:24.293 NETFNET[2503:976392] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFSet addObject:]: attempt to insert nil'</p>
  </blockquote>
  
  <p>I am trying to access Data Base simultaneously, I think, from main and background threads. I have seen a lot of solutions for Objective C, but none for Swift (I don't know Objective C...). Unfortunately, I don't know how to work with Grand Central Dispatch and, in fact, my program does not really need several treads (I mean it need it, but if some thread loses info from one function for one time, nothing bad will happen). I just want to have stable program on Swift 1 or 2, so I will be thankful for any help.</p>