connectivity handling, audio publishing

lib/src/participant/local_participant.dart

@@ -71,6 +71,9 @@ class LocalParticipant extends Participant {
     var trackInfo = await _engine.addTrack(
         cid: track.getCid(), name: track.name, kind: track.kind);
     var stream = await getMediaStream();
+    if (stream == null) {
+      return Future.error(TrackPublishError());
+    }
     var transceiverInit = new RTCRtpTransceiverInit(
       direction: TransceiverDirection.SendOnly,
       streams: [stream],