asp.net2.0的treeview用下面的脚本进行checkbox级联，老提示缺少对象？

<script type= "text/javascript ">

function public_GetParentByTagName(element, tagName)
{
var parent = element.parentNode;
var upperTagName = tagName.toUpperCase();
//如果这个元素还不是想要的tag就继续上溯
while (parent && (parent.tagName.toUpperCase() != upperTagName))
{
parent = parent.parentNode ? parent.parentNode : parent.parentElement;
}
return parent;
}

//设置节点的父节点Cheched——该节点可访问，则他的父节点也必能访问
function setParentChecked(objNode)
{
var objParentDiv = public_GetParentByTagName(objNode, "div ");
if(objParentDiv==null || objParentDiv == "undefined ")
{
return;
}
var objID = objParentDiv.getAttribute( "ID ");
objID = objID.substring(0,objID.indexOf( "Nodes "));
objID = objID+ "CheckBox ";
var objParentCheckBox = document.getElementById(objID);
if(objParentCheckBox==null || objParentCheckBox == "undefined ")
{
return;
}
if(objParentCheckBox.tagName!= "INPUT " && objParentCheckBox.type == "checkbox ")
return;
objParentCheckBox.checked = true;
setParentChecked(objParentCheckBox);
}

//设置节点的子节点uncheched——该节点不可访问，则他的子节点也不能访问
function setChildUnChecked(divID)
{
var objchild = divID.children;
var count = objchild.length;
for(var i=0;i <objchild.length;i++)
{
var tempObj = objchild[i];
if(tempObj.tagName== "INPUT " && tempObj.type == "checkbox ")
{
tempObj.checked = false;
}
setChildUnChecked(tempObj);
}
}

//设置节点的子节点cheched——该节点可以访问，则他的子节点也都能访问
function setChildChecked(divID)
{
var objchild = divID.children;
var count = objchild.length;
for(var i=0;i <objchild.length;i++)
{
var tempObj = objchild[i];
if(tempObj.tagName== "INPUT " && tempObj.type == "checkbox ")
{
tempObj.checked = true;
}
setChildChecked(tempObj);
}
}

//触发事件
function CheckEvent()
{

var objNode = event.srcElement;

if(objNode.tagName!= "INPUT " || objNode.type!= "checkbox ")
return;

if(objNode.checked==true)
{
setParentChecked(objNode);
var objID = objNode.getAttribute( "ID ");
var objID = objID.substring(0,objID.indexOf( "CheckBox "));
var objParentDiv = document.getElementById(objID+ "Nodes ");
if(objParentDiv==null || objParentDiv == "undefined ")
{
return;
}
setChildChecked(objParentDiv);
}
else
{
var objID = objNode.getAttribute( "ID ");
var objID = objID.substring(0,objID.indexOf( "CheckBox "));
var objParentDiv = document.getElementById(objID+ "Nodes ");
if(objParentDiv==null || objParentDiv == "undefined ")
{
return;
}
setChildUnChecked(objParentDiv);
}
}

</script>


<asp:TreeView ID="TreeView1" runat="server" ShowCheckBoxes="All" ExpandDepth="0" BorderWidth="0px" EnableClientScript="true">
</asp:TreeView>

后台加：TreeView1.Attributes.Add("onclick","CheckEvent()");