After deleting an image from the web interface, the /api/user/<nickname>/{inbox,feed} feeds returns http error 500

Error in log:

Error - <type 'exceptions.AttributeError'>: 'NoneType' object has no attribute 'serialize'
URL: http://mediagoblin.jonash.local/api/user/kabniel/feed
File '/srv/mediagoblin.local/mediagoblin/local/lib/python2.7/site-packages/Paste-1.7.5.1-py2.7.egg/paste/exceptions/errormiddleware.py', line 144 in __call__
  app_iter = self.application(environ, sr_checker)
File '/srv/mediagoblin.local/mediagoblin/mediagoblin/app.py', line 268 in __call__
  return self.call_backend(environ, start_response)
File '/srv/mediagoblin.local/mediagoblin/local/lib/python2.7/site-packages/Werkzeug-0.9.6-py2.7.egg/werkzeug/wsgi.py', line 588 in __call__
  return self.app(environ, start_response)
File '/srv/mediagoblin.local/mediagoblin/mediagoblin/app.py', line 245 in call_backend
  response = controller(request)
File '/srv/mediagoblin.local/mediagoblin/mediagoblin/decorators.py', line 427 in wrapper
  return controller(request, *args, **kwargs)
File '/srv/mediagoblin.local/mediagoblin/mediagoblin/federation/views.py', line 359 in feed_endpoint
  feed["items"].append(activity.serialize(request))
File '/srv/mediagoblin.local/mediagoblin/mediagoblin/db/mixin.py', line 455 in serialize
  "object": self.get_object.serialize(request),
AttributeError: 'NoneType' object has no attribute 'serialize'

The recent change to allow for footer adjustment in #952 doesn't work automatically on all themes (see fix in commit aa68730 to see this fixed in airy). Themes now require specifying how high their footer is and adjusting the "wrap" div to offset that height.

sandyseventiesspeedboat doesn't have this change yet.

Additionally if an update could be sent to the list informing theme authors, that would be great.

From ​https://mediagoblin.readthedocs.org/en/latest/pluginwriter/api.html#plugin-api-chapter

"The plan at present is that we are adding hooks as people need them, with community discussion. If you find that you need a hook and MediaGoblin at present doesn’t provide it at present, please ​http://mediagoblin.org/pages/join.html! We’ll evaluate what to do from there."

The api.rst file has the following text.

The plan at present is that we are adding hooks as people need them,
with community discussion.  If you find that you need a hook and
MediaGoblin at present doesn't provide it at present, please
`http://mediagoblin.org/pages/join.html <talk to us>`_!  We'll
evaluate what to do from there.

Obviously the link text is supposed to say "talk to us" and point to join.html, but apparently the rst format is incorrect.