Notification.prototype = {
  /**
   * Removes the notification and updates the popup accordingly if needed.
   */
  remove: function Notification_remove() {
    this.owner.remove(this);
  },

  get anchorElement() {
    if (!this.owner.iconBox)
      return null;

    let anchorElement = null;
    if (this.anchorID) // вся проблема - в строчке ниже:
      anchorElement = this.owner.iconBox.querySelector("#"+this.anchorID);

    if (!anchorElement)
      anchorElement = this.owner.iconBox;

    return anchorElement;
  }
};