把鼠标放到二级菜单里的‘li’就会出现报错？

<script>
window.onload=function(){
    var aLi=document.getElementsByTagName('li');   
    for(var i=0; i<aLi.length; i++){
       aLi[i].onmouseover=function(){
            var This=this;
             var subnav=This.getElementsByTagName('ul')[0];
            This.time=setInterval(function(){ 
                if(subnav.offsetHeight>=120){
                    subnav.style.height=120+'px';
                    clearInterval(This.time);   
                }
                subnav.style.height=subnav.offsetHeight+12+"px";
                   
                }
                ,
                30)
        }
        //鼠标离开菜单，二级菜单动画收缩起来。        
        aLi[i].onmouseout=function(){
            var This=this;
             var subnav=This.getElementsByTagName('ul')[0];
            This.time=setInterval(function(){ 
                if(subnav.offsetHeight<=0){
                    subnav.style.height=0+'px';
                    clearInterval(This.time);   
                }
                subnav.style.height=subnav.offsetHeight-12+"px";
                }
                ,
                30)
        }     
    }
}
</script>