AttributeError at /admin/r/31/4/ 'NoneType' object has no attribute 'startswith'

Hello, everyone. I was making something with janggo and got stuck, but I don't understand why the error message came out, so I'm asking if anyone knows. I Googled and tried it, but I really didn't get it.

to sum up the situation models.py: Create a collection in the form of a post and a corresponding category. Both category and model created get_absolute_url, and category loaded well. However, the model can be displayed, but the click itself cannot be clicked on the screen when put in HTML. views.py: class LectureDetail(DetailView): model = Lecture def lecture_detail(request, id): lecture = get_object_or_404(Lecture, id=id) return render(request, 'instructor/lecture_detail.html', {'lecture':lecture})

In this way, I created two apps with almost the same view and model, but I can't click both on the screen ({l.get_absolute_url}}). If you look at the detail page through the admin, only one side of the error appears.

I don't know if it's because the model name is duplicated. But to do that, there was no problem putting one category model into other apps?

I'm sorry for the question. --;;