Hi all, I followed the guide ​http://docs.mediagoblin.org/siteadmin/media-types.html#video to play videos, but when I try to load an mp4 video celeryd gives me this traceback

2013-06-21 08:35:06,303 INFO    [celery] Got task from broker: mediagoblin.processing.task.ProcessMedia[79d48707-aeee-46d7-85be-f26fc5f35ae7]
2013-06-21 08:35:13,030 WARNING [celery] Traceback (most recent call last):
2013-06-21 08:35:13,032 WARNING [celery] File "/media/LaCie/home/mediagoblin/thompson-mediagoblin-2.6/mediagoblin/media_types/video/transcoders.py", line 458, in __on_discovered
2013-06-21 08:35:13,063 WARNING [celery] raise Exception('Could not discover {0}'.format(self.source_path))
2013-06-21 08:35:13,065 WARNING [celery] Exception
2013-06-21 08:35:13,066 WARNING [celery] :
2013-06-21 08:35:13,068 WARNING [celery] Could not discover /home/mediagoblin/thompson-mediagoblin-2.6/user_dev/media/queue/media_entries/79d48707-aeee-46d7-85be-f26fc5f35ae7/Lolo__Show__Sulla_neve.mp4
2013-06-21 08:35:13,721 ERROR   [celery] Task mediagoblin.processing.task.ProcessMedia[79d48707-aeee-46d7-85be-f26fc5f35ae7] raised exception: TypeError("argument of type 'NoneType' is not iterable",)
Traceback (most recent call last):
  File "/media/LaCie/home/mediagoblin/thompson-mediagoblin-2.6/lib/python2.6/site-packages/celery-2.5.3-py2.6.egg/celery/execute/trace.py", line 181, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/media/LaCie/home/mediagoblin/thompson-mediagoblin-2.6/mediagoblin/processing/task.py", line 92, in run
    entry.media_manager.processor(proc_state)
  File "/media/LaCie/home/mediagoblin/thompson-mediagoblin-2.6/mediagoblin/media_types/video/processing.py", line 91, in process_video
    store_metadata(entry, metadata)
  File "/media/LaCie/home/mediagoblin/thompson-mediagoblin-2.6/mediagoblin/media_types/video/processing.py", line 170, in store_metadata
    if key in metadata])
TypeError: argument of type 'NoneType' is not iterable

I'm currently using mediagoblin 0.4.1.dev in the thompson 421-mysql branch.

To reproduce:

• Go to a media page
• Click "Add media to collection"
• On the resulting page, don't select a collection from the dropdown so it stays on "-- Select --"
• Click "Add"
• MediaGoblin crashes with this message: "AttributeError: 'NoneType' object has no attribute 'id'"

Possible options:

• Return an error when no collection is specified
• Remove the "-- Select --" option from the dropdown (thereby automatically selecting a certain collection - maybe the default "Favorites" collection we talked about?)