set onmessage(fn) {
    var time = this.method.microSeconds();
    var listenObj = {
      time: time,
      fn: fn
    };
    _removeListenerObject(this, 'message', this._onML);
    if (fn && typeof fn === 'function') {
      this._onML = listenObj;
      _addListenerObject(this, 'message', listenObj);
    } else {
      this._onML = null;
    }
  }
In the past when this error appeared, it was really hard to debug.
So now we log the msg together with the error so it at least
gives some clue about where in your application this happens.