removeBackNodes(duration) {
    if (this.backNode != null) {
      if (duration == null) { duration = 0.0; }
      this.removeInteractiveElement(this.backNode);
      this.backNode.destroy(duration);
      this.backNode = null;
    }
  }
Removes current back node(s).
@param {Number} [duration=0.0] time in seconds