code: function() {
  
              this.firstElement = null;
              this.lastElement = null;
  
              var elements = this.getFocusableElements();
              this.focusableElements = elements;
  
              if (elements.length > 0) {
                  this.firstElement = elements[0];
                  this.lastElement = elements[elements.length - 1];
              }
  
              if (this.cfg.getProperty("modal")) {
                  this.setTabLoop(this.firstElement, this.lastElement);
              }
          }
docs: Sets the firstElement and lastElement instance properties
  to the first and last focusable elements in the Panel.
  
  @method setFirstLastFocusable