AJAX :: Progress Indicator In Modal Popup Window?

I'm trying to use the modal popup window extender to display a 'loading' icon when the user uploads a file. I'm using the following:

Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginReq);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endReq);
function beginReq(sender, args)
{
// shows the Popup
$find(ModalProgress).show();
}
function endReq(sender, args)
{
// shows the Popup
$find(ModalProgress).hide();
}
//END --
// on the .aspx page
<script type="text/javascript" language="javascript" >
var ModalProgress = '<%= modalProgress.ClientID %>';
</script>
<asp:Panel
ID="pnlModal"
CssClass="modalPopup"
Style="display: none"
Width="233px"
runat="server" >
<img alt="Loading Icon" src="../images/redLoading.gif" />

The file is being uploaded, depending on the number of records this may take up to a minute.

</asp:Panel>

which I found on Berseth's site but I cannot get it to work. 'Null' error when it's trying to pop the window. Does anyone out here have another way to do this.