message: Initialize event listener to nullptr
diff: @@ -51,6 +51,7 @@ MediaStream::MediaStream(std::shared_ptr<Worker> worker,
  const std::string& media_stream_label,
  bool is_publisher) :
  audio_enabled_{false}, video_enabled_{false},
  + media_stream_event_listener_{nullptr},
  connection_{std::move(connection)},
  stream_id_{media_stream_id},
  mslabel_ {media_stream_label},
  