macOS Threading of Core Data in a document based application

[bitdoctor]

Hi folks,
I am hanging around for a while with a multithreading core data app. As Apple remarks, managed object contexts are not threadsave, they suggest to create an own context for each thread. There some code snippets about that in the internet. However, they all do not match my problem.

My Thread creates a new context

  // Create context on background thread
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
    [context setUndoManager:nil];
    [context setPersistentStoreCoordinator: [ipDocument.managedObjectContext persistentStoreCoordinator]];
    
    // Register context with the notification center
    NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
    [nc addObserver:self
           selector:@selector(mergeChanges:)
               name:NSManagedObjectContextDidSaveNotification
             object:context];

the according -merge: method is as following:

- (void)mergeChanges:(NSNotification *)notification
{
    NSLog(@"merge managed contexes");
    NSManagedObjectContext *mainContext = [ipDocument managedObjectContext];
    
    // Merge changes into the main context on the main thread
    [mainContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                                  withObject:notification
                               waitUntilDone:YES];
}

The context is created properly and [ipDocument.managedObjectContext persistentStoreCoordinator] returns a value not NULL. However, when saving the document and forcing to merge the contexts by

NSError *error = nil;
        [context save:&error];
        
        if (error)
        {
            [NSApp presentError:error];
        }
        
        [context reset];

the application crashes with
An uncaught exception was raised
This NSPersistentStoreCoordinator has no persistent stores. It cannot perform a save operation.

As I said, this is a document based app and the matter happens short time after starting the app (with a new, empty document). What is wrong with this? Thank you in advance