搞不懂?为什么会这样？

Question

index.html:73 Uncaught TypeError: Cannot read property 'getElementsByTagName' of null    at window.onload (index.html:73)？按照老师的的写法,出现上面的错误提示<script>   function $(id){      return typeof id==='string'?document.getElementById(id):id;   }   window.onload=function(){      var titles=$('notice-tit').getElementsByTagName('li');<script>