class GroupMembership(models.Model):
    group = models.ForeignKey('group.Group', related_name='members', on_delete=models.CASCADE)
    member = models.ForeignKey(
        'group.Member',
        related_name='group_membership',
        on_delete=models.CASCADE,
    )

class GroupMembershipRequest(models.Model):
    group = models.ForeignKey('group.Group', related_name='requests',  on_delete=models.CASCADE)
    member = models.ForeignKey(
        'group.Member',
        related_name='group_membership_request',
        on_delete=models.CASCADE,
    )

AttributeError: 'NoneType' object has no attribute '_state'