destructor() {
    this._observer.disconnect();
    this._observer = null;

    if (this._rootElement)
      this._rootElement.removeAttribute('aria-hidden');
    this._rootElement = null;

    for (let inertNode of this._managedNodes)
      this._unmanageNode(inertNode.node);

    this._managedNodes = null;

    this._inertManager = null;
  }
Call this whenever this object is about to become obsolete.  This unwinds all of the state
stored in this object and updates the state of all of the managed nodes.