message: don't return invalid indices from model, fix for
diff: @@ -80,7 +80,8 @@ export default class EditorModel {
  const part = this._parts[index];
  return new DocumentPosition(index, part.text.length);
  } else {
  - return new DocumentPosition(0, 0);
  + // part index -1, as there are no parts to point at
  + return new DocumentPosition(-1, 0);
  }
  }
  