急救，高手，Jquery easyui不能觸發後臺事件

[康少_小贱]

Microsoft JScript runtime error: Unable to set value of the property 'value': object is null or undefined

var theForm = document.forms['form1'];
if (!theForm) {
theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
theForm.__EVENTTARGET.value = eventTarget;
theForm.__EVENTARGUMENT.value = eventArgument;
theForm.submit();
}
}

是使用jquery easyui layout 框架，導致不能觸發dropdownlist selectindexchange事件
我在前臺寫了一個dropdownlist onchange，也有服務器事件selectindexchange，不能觸發selectindexchagne事件，onchange事件會報上面的js runtime error錯誤。