'NoneType'对象没有属性'unique'但我没有使用'unique'属性

I'm build models in Django 1.8, and I'm using abstract inheritance (which I'm assuming is contributing to the problem). I have abstract models and then I have models which are based on those abstract models. I also have ForeignKey and ManyToMany relations between some models.I'm build models in Django 1.8, and I'm using a