how to resolve run time error in modal pop up extender in asp.net

<pre lang="xml"><asp:Panel ID="Panel1" runat="server"  Width="500px"
      Font-Size="Medium" CssClass="confirm-dialog" BackColor="#99CCFF">

      <asp:Panel ID="Panel2" runat="server" BackColor="White" Height="25px">
          <asp:ImageButton ID="ImageButton1" runat="server"
              ImageUrl="~/images/cross.png" Width="40px" Height="25px" />
      </asp:Panel>  </asp:Panel>

<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="Panel1" TargetControlID="Button1" BackgroundCssClass="modalBackground">
  </asp:ModalPopupExtender>

.modalBackground {


    background-color:Silver;
    filter:alpha(opacity=70);opacity:0.7;


}

<asp:Button ID="Button1" runat="server" Text="Button" style="display:none"/>

<script type="text/javascript">
//<![CDATA[
Sys.Application.add_init(function() {
    $create(Sys.Extended.UI.ModalPopupBehavior, {"BackgroundCssClass":"modalBackground","PopupControlID":"ContentPlaceHolder1_Panel1","dynamicServicePath":"/job_response.aspx","id":"ContentPlaceHolder1_ModalPopupExtender1"}, null, null, $get("ContentPlaceHolder1_Button1"));
});
//]]>
</script>