local: Filter out None from get_distinct results

Fixes #1202

docs/changelog.rst

@@ -80,6 +80,13 @@ MPD frontend
 - Track data now include the ``Last-Modified`` field if set on the track model.
   (Fixes: :issue:`1218`, PR: :issue:`1219`)
 
+Local backend
+-------------
+
+- Filter out :class:`None` from
+  :meth:`~mopidy.backend.LibraryProvider.get_distinct` results. All returned
+  results should be strings. (Fixes: :issue:`1202`)
+
 File backend
 ------------
 

mopidy/local/json.py

@@ -174,7 +174,7 @@ class JsonLibrary(local.Library):
         search_result = search.search(self._tracks.values(), query, limit=None)
         for track in search_result.tracks:
             distinct_result.update(distinct(track))
-        return distinct_result
+        return distinct_result - {None}
 
     def search(self, query=None, limit=100, offset=0, uris=None, exact=False):
         tracks = self._tracks.values()