Race in DistributionWithContentFilter can lead to 500

[mdellweg]

The only possible explanation I can find is that cast is messing up the related prefetched models, while acutally not be needed.
All we need is the class of the distribution. Not the detail instance.

    for dist in qs.exclude(repository=None).prefetch_related("repository__versions"):
        if dist.cast().SERVE_FROM_PUBLICATION:
            versions = dist.repository.versions.values_list("pk", flat=True)
            ....

pulp [c19d4bd0-3b4c-4d3a-87fb-9cd2b5c2edd2]: django.request:ERROR: Internal Server Error: /pulp/api/v3/distributions/file/file/
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.8/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/rest_framework/viewsets.py", line 125, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
  File "/usr/local/lib/python3.8/site-packages/rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/usr/local/lib/python3.8/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "/usr/local/lib/python3.8/site-packages/rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/rest_framework/mixins.py", line 38, in list
    queryset = self.filter_queryset(self.get_queryset())
  File "/usr/local/lib/python3.8/site-packages/rest_framework/generics.py", line 150, in filter_queryset
    queryset = backend().filter_queryset(self.request, queryset, self)
  File "/usr/local/lib/python3.8/site-packages/django_filters/rest_framework/backends.py", line 74, in filter_queryset
    return filterset.qs
  File "/usr/local/lib/python3.8/site-packages/django_filters/filterset.py", line 230, in qs
    qs = self.filter_queryset(qs)
  File "/usr/local/lib/python3.8/site-packages/django_filters/filterset.py", line 213, in filter_queryset
    queryset = self.filters[name].filter(queryset, value)
  File "/usr/local/lib/python3.8/site-packages/pulpcore/app/viewsets/custom_filters.py", line 424, in filter
    versions = dist.repository.versions.values_list("pk", flat=True)
AttributeError: 'NoneType' object has no attribute 'versions'