Document.GetElementByID returning undefined for gridview {...}

See more:

C#

ASP.NET

Javascript

I am working on a page which has a master page. I have a gridview in the page and inside it I am dynamically populating radio buttons. OnClick event of radio button I am calling a javascript function which is in some other file.

<asp:GridView AutoGenerateColumns="False" AllowSorting="false" Width="100%" ID="List" runat="server" AllowPaging="false" GridLines="None" ShowHeader="False">
<Columns>
<asp:TemplateField ItemStyle-HorizontalAlign="Left">                                         
<ItemTemplate>
<asp:RadioButton ID="Criteria" Width="1px" runat="server" GroupName="SelectCriteria" onclick="javascript:ValidateMultipleCheck(this,'CPH_List');" />
<asp:HiddenField ID="hdn" runat="server" Value='<%# DataBinder.Eval (Container.DataItem, "Key") %>' />>'

</ItemTemplate>
</asp:TemplateField>

<asp:BoundField DataField="Value" ItemStyle-HorizontalAlign="Left" />
</Columns>
</asp:GridView>

function ValidateMultipleCheck(control, gridName) {
    var controlId = control.id;
    var Parent = document.getElementById(gridName);
    for (i = 0; i < Parent.rows.length; i++) {
        var tr = Parent.rows[i];
        var td = tr.childNodes[0].childNodes[0];
        var item = td.firstChild;
        if (item != null && item.id != controlId && item.type == "radio") {
            if (item.checked) {
                item.checked = false;
            }
        }
    }
}

While I click on the radio buttons, in the javascript function, though I am getting the values passed correctly but the var Parent's value is showing as {...} and later terminating the execution as unable to get firstchild for undefined or null node.

Please let me know what I am missing here.