message: checking if activeSources still exists before trying to remove elements
  fixes
diff: @@ -149,10 +149,12 @@ define(["../core/Tone", "../instrument/Instrument", "../core/Buffers", "../sourc
  //remove it when it's done
  source.onended = function(){
  + if (this._activeSources && this._activeSources[midi]){
  var index = this._activeSources[midi].indexOf(source);
  if (index !== -1){
  this._activeSources[midi].splice(index, 1);
  }
  + }
  }.bind(this);
  }
  }
  