Internal Server Error

The server has either erred or is incapable of performing the requested operation.

Traceback (most recent call last):
  File "backend/lib/webapp2.py", line 1536, in __call__
    rv = self.handle_exception(request, response, e)
  File "backend/lib/webapp2.py", line 1530, in __call__
    rv = self.router.dispatch(request, response)
  File "backend/lib/webapp2.py", line 1278, in default_dispatcher
    return route.handler_adapter(request, response)
  File "backend/lib/webapp2.py", line 1102, in __call__
    return handler.dispatch()
  File "backend/iaa/handlers.py", line 60, in dispatch
    webapp2.RequestHandler.dispatch(self)
  File "backend/lib/webapp2.py", line 572, in dispatch
    return self.handle_exception(e, self.app.debug)
  File "backend/lib/webapp2.py", line 570, in dispatch
    return method(*args, **kwargs)
  File "backend/iaa/handlers.py", line 155, in model_page
    instance.url = str(instance.url).replace("http://", "https://")
AttributeError: 'NoneType' object has no attribute 'url'