'this.el.dom.value.length'为空或不是对象，怎么回事？

[code="js"]

Ext.onReady(function(){ Ext.BLANK_IMAGE_URL='ExtJS2/resources/images/default/s.gif'; var form = new Ext.form.TriggerField({ title:'Ext.form.TriggerField示例', labelSeparator:':', labelWidth:80, bodyStyle:'padding:5 5 5 5', frame:true, height:100, width:270, applyTo:'form', items:[ new Ext.form.TriggerField({ id:'memo', fieldLabel:'触发字段', hideTrigger:false, onTriggerClick:function(e){ var memo = form.findById('memo'); alert(memo.getValue()); } }) ] }) });

[/code]
页面中已经有"form”这个元素了呀,为什么还提示这个错误?

[code="js"]var form = new Ext.form.FormPanel({ //不是var form = new Ext.form.TriggerField({ [/code]