Fix 500 error at delete page with PlaceholderField in use.
Fix 500 error at delete page with PlaceholderField in use.

The forward-relation of the reverse-relation needs to exist,
however it is not that important right now. Leave it as None for the
moment.

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