Fix rendering cache if there are stale base classes in the database.
Fix rendering cache if there are stale base classes in the database.

Not using the object indexes of the base queryset anymore,
because the derived object data may not exist.

django-fluent/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,ixc/django-fluent-contents,ixc/django-fluent-contents,jpotterm/django-fluent-contents,edoburu/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,pombredanne/django-fluent-contents,jpotterm/django-fluent-contents,ixc/django-fluent-contents,pombredanne/django-fluent-contents,pombredanne/django-fluent-contents,jpotterm/django-fluent-contents