Prevent null pointer on music.spotify backend stop if the Librespot process has already been terminated

platypush/backend/music/spotify/__init__.py

@@ -204,6 +204,7 @@ class MusicSpotifyBackend(Backend, SpotifyMixin):
 
                 while not self.should_stop():
                     try:
+                        if self._librespot_proc:
                             self._librespot_proc.wait(timeout=1.0)
                     except subprocess.TimeoutExpired:
                         pass