AttributeError: 'GenericForeignKey' object has no attribute 'null'

I'm getting a very weird error when I try any method (delete, save, etc) on a model that holds a Generic Foreign Key. My model can hold different type of card types, and a job to process them:

class JobCards(models.Model):

    class Meta:
        unique_together = ('content_type', 'object_id')

    content_type = models.ForeignKey(
        ContentType,
        on_delete=models.CASCADE,
        help_text=_('Card type')
    )
    object_id = models.PositiveIntegerField(
        help_text=_('Card id')
    )
    content_object = GenericForeignKey()

    job = models.ForeignKey(
       Job,
       on_delete=models.CASCADE
    )

So operations like:

JobCards.objects.create(content_object=card, job=job)

or

job_card = JobCards.objects.get(
              job=job,
              content_type=ContentType.objects.get_for_model(card),
              object_id=card.id
           )
job_card.delete()

will fail with such error.

I should mention that I put two GenericRelation in the card models, but even removing them doesn't change the end result:

job_cards = GenericRelation(JobCards, related_query_name='card')

Solved. Leaving this here just in case someone comes across the same issue. The problem was project related, not an issue with Django. The ContentType models I was using required a specific field, and somehow with a generic foreign key that is lost (while it seems just fine with a normal foreign key). Adding it as a property of the model did the trick:

@property
def required_field(self):
    return self.content_object.some_linked_model.required_field