app调试的时候，出现错误

 Uncaught TypeError: Cannot read property 'style' of undefined at js/function.js:44
 Uncaught TypeError: Cannot read property 'style' of undefined at js/function.js:44

这个是什么意思

 42  function ShowLoginPage() {
 43 DOM['top'].style.display = 'none';
 44 DOM['nav'].getElementsByTagName("div")[1].style.display = 'none';    //这里提示出错
 45 DOM['nav'].getElementsByTagName("div")[0].style.display = 'block';
 46 DOM['div'].style.height = DOM['body'].clientHeight - 49 + 'px';
 47 DOM['iframe'].src = 'login.html';
 //关闭所有临时窗口
 $('.onepage').remove();
}