code: function () {
              Overlay.windowResizeEvent.unsubscribe(this.sizeMask, this);
              this.removeMask();
              if (this.close) {
                  Event.purgeElement(this.close);
              }
              Panel.superclass.destroy.call(this);  
          }
docs: Removes the Panel element from the DOM and sets all child elements
  to null.
  @method destroy