fix: MusicCast - non-existing player in zone handling (#2606)

Author: fmunkes

music_assistant/providers/musiccast/player.py

@@ -336,7 +336,9 @@ async def _handle_zone_grouping(self, zone_player: MusicCastZoneDevice) -> None:
         # verify that this source actually exists and is non net
         _allowed_sources = self._get_allowed_sources_zone_switch(zone_player)
         mass_player = self.mass.players.get(player_id)
-        assert mass_player is not None
+        if mass_player is None:
+            # Do not assert here, should the player not yet exist
+            return
         if _source not in _allowed_sources:
             msg = (
                 "The switch source you specified for "