在ie11下调试，报出：0x800a138f - JavaScript 运行时错误: 无法获取未定义或 null 引用的属性
<script language="javascript"> 
function initializeMenu(menuName) { 
  var    menu= document.getElementsByName(menuName); 
  var k,i,len,tablelen; 
  for     ( k=0,tablelen=menu.length; k <tablelen;     k++)         
  { 
    var actuator=menu[k]; 
    actuator.onclick = function() { 
                var display = this.nextSibling.nextSibling.style.display; 
    if (display == "none") 
      this.nextSibling.nextSibling.style.display = "block"; 
    else 
      this.nextSibling.nextSibling.style.display = "none"; 
                return false;} 
        } 
} 
function showcategory(menuID) 
{ 
  var menu=document.getElementById(menuID); 
  //menu.className=""; 
  while (menu.parentElement.id!="MenuRoot") 
  {     
    if (menu.tagName=="UL" && menu.style.display=='none') 
    { 
      menu.style.display="block"; 
      //menu.className=""; 
    } 
    menu=menu.parentElement; 
  } 
} 
window.onload = function() { 
     initializeMenu("menuname"); 
} 
</script> 
<div id="MenuRoot"> 
    <ul> 
        
      <ul> 
          <li><a href="#" name="menuname" id="1">第一级菜单</a> 
                <ul style="display:none"> 
                  <li><a href="#" >第二级菜单3</a></li> 
                  <li><a href="#">第二级菜单4</a></li> 
                  <li><a href="#">第二级菜单5</a></li> 
              </ul> 
          </li> 
          
          <li><a href="" name="menuname" title="" id="2">Flashlights</a>
               <ul style="display:none">
                <li><a href="">Rechargeable Flashlights</a></li>
                <li><a href="" >LED Flashlights & Sets</a></li>
               </ul>
         </li> 

      </ul> 

    </ul> 
</div> 
<script>showcategory("2");</script>