stopShowingChapter(duration) {
    if (duration == null) { duration = 0.0; }

    if (this._bg != null) {
      this._bg.destroy(duration);
      this._bg = null;
    }

    return Promise.resolve();
  }
Stops showing a chapter and returns a promise.
@param {Number} [duration=0.0] duration to remove over
@returns {Promise}