ModalPopupExtender show method does not work from code-behind javascript

Question

I have an ajax modal popupextender which I would like to show using java script at the end of some processing in the code behind. I get the message Error: Unable to get property 'show' of undefined or null reference.

<asp:Panel ID="panel1" runat="server" Visible="true" BorderColor="Black" Style="display: none">
 <asp:UpdatePanel ID="uppanel1" runat="server">
  <ContentTemplate>
    <asp:Button ID="btn1" Text="Popup" Visible="true" runat="server" Style="display:none"/>    
    <ajaxToolKit:ModalPopupExtender ID="mpe1" runat="server" TargetControlID="btn1"     PopupControlID="panel1" RepositionMode="None"  PopupDragHandleControlID="drag1" BehaviorID="behave1"/>               
     <div id="div1" runat="server">                    
       <asp:TextBox ID="txt1" runat="server" Text="Text" ></asp:TextBox> 
     </div>    
  </ContentTemplate>
 </asp:UpdatePanel>
</asp:Panel>
function ShowPopUp(mpid) {
  var id1 = $find(mpid);
  id1.show();
}

Answer 1

Try this

$find("<%= mpe1.ClientID %>").show();