Export Excel option in Update Panel.(Throwing Excpetion)

I am using the same code as I discuss earlier here http://engineerhacks.blogspot.ae/2017/07/converting-gridview-in-to-excel-or.html
 but this code is throwing exception every time "Unable to evaluate expression". I wondering
how it is possible that this code is running on the other pages and in this page is throwing exception when I cross check with other page I saw
that in my current page some one put Update Panel....
So if you are in same condition just put after the ContentTemplate end tag

<Triggers>

        <asp:PostBackTrigger ControlID="btnExcel" />

    </Triggers>

btnExcel is my button id when i click to generate the excel