Solved: ModalPopupExtender Javascript Error on Cancel

I have an asp.net content web page in a master page. The update works, but when I click the cancel button, i get an error - "Microsoft JScript runtime error: 'null' is null or not an object". The variable "modal" is null.

Any ideas?

<%@ Page Language="C#" MasterPageFile="~/secure/SecureMasterPage.master" AutoEventWireup="true" Inherits="secure_Test" Title="Untitled Page" Codebehind="Test.aspx.cs" %>
<%@ Register Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Web.UI" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <script language="javascript" type="text/javascript">
        function fnClickUpdate(sender, e) 
        { 
          __doPostBack(sender,e); 
        }
        function HideModalPopup() 
        { 
          var modal = $find('ModalPopupExtender1.ClientID'); 
          modal.hide(); 
        }
    </script>
 
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:Label ID="lblSomething" runat="server"  CssClass="bcm" Text="nothing has been done" />
    <br />
    <asp:LinkButton runat="server" CssClass="bcm" ID="lbn1" Text="Click to change"></asp:LinkButton>
    <asp:Panel runat="server" ID="pnlModalPopup" CssClass="modalPopup">
        <asp:Label runat="server" ID="lblModalDisplay" CssClass="bcmBig"></asp:Label>
        <br />
        <br />
        <asp:Button runat="server" ID="btnUpdate" Text="Update" 
            onclick="btnUpdate_Click" />
        <asp:Button runat="server" ID="btnCancel" Text="Cancel" />
        
    </asp:Panel>
    
        <ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
            TargetControlID="lbn1"
             PopupControlID="pnlModalPopup"
              BackgroundCssClass="modalBackground"
               DropShadow="true"
                 CancelControlID="btnCancel"
                   OnCancelScript="HideModalPopup()">
        </ajaxToolkit:ModalPopupExtender>
    
</asp:Content>

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