code: protected JTree createTree() {
      JTree tree = new JTree(model);
      tree.setName("TreePopup.tree");
      tree.setFont(getFont());
      tree.setForeground(getForeground());
      tree.setBackground(getBackground());
      tree.setBorder(null);
      tree.setFocusable(true);
      tree.addMouseListener(handler);
      tree.addKeyListener(handler);
      tree.setCellRenderer(new Renderer());
      return tree;
    }
docs: Creates the JList used in the popup to display the items in the combo box
  model. This method is called when the UI class is created.
  
  @return a <code>JList</code> used to display the combo box items