ModalPopUpExtender / GridView / AJAX undefined error in IE8 and IE9

I use a ModalPopUpExtender to display a user control containing a gridview. When a row within the gridview is clicked, the corresponding function in the user control raises a function which closes the popupextender, as follows:    <asp:Button ID="btHidden" runat="server" Text="hidden" style="display: none;" />         <!-- pop up window for patient list -->         <asp:ModalPopupExtender ID="mdChoosePatient" runat="server" TargetControlID="btHidden"             PopupControlID="pnChoosePatient" backgroundcssclass="modalBackground" DropShadow="false">             <Animations>        &nb ...