ext提示：'this.el.dom' 为空或不是对象，请高手帮忙，非常感谢（在线等）

我的采用ext的后台管理框架，偶尔会出现'this.el.dom' 为空或不是对象错误，但是在火狐下又是正常的，请高手们帮我解决下，不胜感激
代码： <link href="ext/resources/css/ext-all.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript" src="ext/adapter/ext/ext-base.js"></script>
    <script type="text/javascript" src="ext/ext-all.js"></script>
    
    <script type="text/javascript">
    Ext.onReady(function(){
       Ext.BLANK_IMAGE_URL="ext/resources/images/default/s.gif";
       var Tree = Ext.tree;
       var tree = new Tree.TreePanel({
            el:'west_content',
            useArrows:true,
            autoHeight:true,
            split:true,
            lines:true,
            autoScroll:true,
            animate:true,
            enableDD:true,
            border:false,
            containerScroll: true, 
            loader: new Tree.TreeLoader({
                dataUrl:'ext_tree_json.aspx' //生成 ext 2.0 所需要的树型格式
                })
        });        // set the root node
        var root = new Tree.AsyncTreeNode({
            text: '影像档案查询',
            draggable:false,
            id:'0' // 0 为根目录
        });
        tree.setRootNode(root);
        // render the tree
        tree.render();
        root.expand();
        
       var viewport = new Ext.Viewport({
            layout:'border',
            items:[ {
                region:'west',
                id:'west',
                //el:'panelWest',
                title:'影像管理系统导航',
                split:true,
                width: 200,
                minSize: 200,
                maxSize: 400,
                collapsible: true,
                margins:'60 0 2 2',
                cmargins:'60 5 2 2',
                layout:'fit',
                layoutConfig:{ activeontop:true},
                defaults: { bodyStyle: 'margin:0;padding:0;'},
                //iconCls:'nav',
                items:
                    new Ext.TabPanel({
                        border:false,
                        activeTab:0,
                        tabPosition:'bottom',
                        items:[{
                                contentEl:'west_content',
                                title:'档案查询',
                                autoScroll:true,
                                bodyStyle:'padding:5px;'
                               },
                               {
                                  layout:'accordion',layoutConfig:{animate:true }, 
                                  title:'后台管理',
                                  autoScroll:true,
                                  border:false,
                                  items:[<%=  GetMenuString() %>]
                               }]
                     })
            },{
                region:'center',
                el:'center',
                deferredRender:false,
                margins:'60 0 2 0',
                html:'<iframe id="center-iframe" width="100%" height=100% name="main"  frameborder="0" scrolling="auto" style="border:0px none; background-color:#BBBBBB; "  ></iframe>',
                autoScroll:true 
            },
            {
                region:'south',
                margins:'0 0 0 2',
                border:false,
                html:'<div class="menu south">书写版权之处</div>'
               }
            ]
        });
        
        setTimeout(function(){
        Ext.get('loading').remove();
        Ext.get('loading-mask').fadeOut({remove:true});
        }, 250)
    });
    </script>
    <base target="_self" />
</head>
<body>
    <form id="form1" runat="server">
     <div id="loading-mask" style=""></div>
      <div id="loading">
        <div class="loading-indicator"><img src="ext/resources/extanim32.gif" width="32" height="32" style="margin-right:8px;" align="absmiddle"/>正在加载，请稍等...</div>
      </div>
  <div id="header"><h1>影像档案管理系统</h1></div>
  <div class="menu">
                <span style="float: left">欢迎：<b>用户名</b>&nbsp;&nbsp;&nbsp; 用户角色：<b>超级管理员</b>
                &nbsp;&nbsp;&nbsp;</span><span id="aLoginOut" runat="server" style="float: right">现在时间:<span id="Calendar"></span>&nbsp;&nbsp;<a onclick="if (!window.confirm('您确认要注消当前登录用户吗？')){return false;}"
                    href="loginout.aspx">注销</a></span>
                    <script src="js/get_time.js" type="text/javascript"></script>
    </div>
  <div id="west">
    
  </div>
  <div id="center">
    
  </div>
  <div id="west_content" style="height:300px; ">
     
  </div>
    </form>
</body>
</html>我自己对ext不是很熟悉，请大家帮帮忙，分数不够可以再加，谢谢大家