Ajax toolkit error - Uncaught Sys.ArgumentNullException: Sys.ArgumentNullException: Value cannot be null. Parameter name: element

 Sys.ArgumentNullException: Sys.ArgumentNullException: Value cannot be null.

Parameter name: element

If you get error in console window in javascript. This is due to below code:-

<ajaxToolkit:ModalPopupExtender ID="popup" BehaviorID="popup" runat="server" DropShadow="false"

PopupControlID="pnlAddEdit" TargetControlID = "lnkFake" BackgroundCssClass="modalBackground" CancelControlID="btnjj" >

</ajaxToolkit:ModalPopupExtender>

You have not declare button for CancelControlID or btn name is wrong.