setFocus() doesn't work on a textbox in a modal window

<zk>
    <button label="show modal" onClick="showModal()"/>
    <zscript><![CDATA[
public void showModal(){
  Window w = new Window();
  w.setPage(page);

  w.appendChild(new Toolbarbutton("get focus"));
  Textbox box1 = new Textbox();
  w.appendChild(box1);
  box1.setFocus(true);
  w.doModal();
}
 ]]></zscript>
</zk>