Handle NoneType with graceful exception

[thclark]

Bug report

About me

If you're new to the twined ecosystem, and you don't mind, it'd be great to hear a little bit of the wider context about
you and what you're working on, so we can understand how best to help.

What is the current behavior?

If self.get_service_revision() is None in the following code, we get an error...

    def ask(self, save=True):
        """Ask a question to a service_revision.

        :param boolean save: If true, this question will be saved in order to update the 'asked' time.
        """

        service_revision = self.get_service_revision()
        subscription, push_url = service_revision.ask(
            question_id=self.id,
            input_values=self.get_input_values(),
            input_manifest=self.get_input_manifest(),
        )

The error is:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/django_gcp/tasks/tasks.py", line 116, in execute
    return self.run(**task_kwargs), 200
  File "/workspace/server/example/tasks.py", line 44, in run
    raise e
  File "/workspace/server/example/tasks.py", line 40, in run
    question.ask()
  File "/workspace/server/example/models.py", line 129, in ask
    subscription, push_url, service_revision = super().ask(save=False)
  File "/usr/local/lib/python3.9/site-packages/django_twined/models/questions.py", line 82, in ask
    subscription, push_url = service_revision.ask(
AttributeError: 'NoneType' object has no attribute 'ask'

What is the expected behavior?

It's a mistake to send a NoneType through to here, but could happen commonly, so a ValueError should be raised if the service_revision is not a valid class instance.