if you look at the code closely. The ModalPopupExtender contain the property CancelControlID="btnNo" but that button was being commented out
<%-- <asp:Button ID="btnNo" runat="server" Text="No" CssClass="no" />--%>
So, to get rid of the error, either remove the CancelControlID="btnNo" from ModalPopupExtender or un-comment the btnNo code.