小弟初学Ext,this.el.dom为空或不是对象

我刚刚开始学习Ext，按着教程做实验的时候，出现了this.el.dom为空 或不是对象的错误信息。代码如下：<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>无标题文档</title>
<link rel="stylesheet" type="text/css" href="extjs/resources/css/ext-all.css">
<script type="text/javascript" src="extjs/ext-base.js" ></script>
<script type="text/javascript" src="extjs/ext-all.js"></script>
<script type="text/javascript">
    var cm = new Ext.grid.ColumnModel([
    {header:'账号',dataindex:'username'},
{header:'密码',dataindex:'passwd'},
{header:'验证',dataindex:'check'}
]);

var data = [
    ['123','ddd','444'],
['345','aaa','555'],
['222','dfw','weq'],
['777','fff','fdw']
];
var ds = new Ext.data.Store({
     proxy: new Ext.data.MemoryProxy(data),
 reader: new Ext.data.ArrayReader({},[
      {name:'username',mapping:2},
  {name:'password',mapping:0},
  {name:'check',mapping:1}
 ])
});
ds.load();
var grid = new Ext.grid.GridPanel({
     el:'grid',
 cm:cm,
 ds:ds

})
grid.render();
</script>
</head><body>
<div id="grid"></div>
</body>
</html>请各位帮忙看看，谢谢大家