Error: Error: document.getElementById("ctl00_cphMain_iframeAddDetail") is null

Question

hi friends,

 I am getting problem in opening iframe into popup screen using modal pop up..\

Following is the code i used.

It give me console error: 

"Error: document.getElementById("ctl00_cphMain_iframeAddDetail") is null"

plz help..

<script type="text/javascript">

function newwindow()

    { 

        $find('mpeAddDetail').show();

        var pageName ="AddSupprtCallDetail.aspx";

        document.getElementById("iframeAddDetail").src = pageName;

        return false;

    }

</script>

--

<cc1:ModalPopupExtender ID="ModalPopupExtender1" TargetControlID="dummylink" CancelControlID="btnCancel"

     BehaviorID="mpeAddDetail" PopupControlID="dialog" DropShadow="true" BackgroundCssClass="modalBackground" runat="server">

 </cc1:ModalPopupExtender>

 

  <div id="dialog" runat="server" style="padding: 10px; width: 820px; color: Gray; font-family: Trebuchet MS; font-size: 13px; background-color: White; border: Solid 20px #FFFFFF;display: none; min-height: 450px; height: 300px;">

        <asp:HiddenField ID="hdnEditContactId" Value="" runat="server"/>

        <table style="margin: 20px; width: 100%">

        <tr>

         <td>

                 <a href = "#" id="dummylink" runat = "server"></a>   

             </td>

            </tr>

           </table>

    </div>

--

  <asp:LinkButton  ID="SupportCall" runat="server" ForeColor = "Blue" Font-Underline ="true" OnClientClick = "return newwindow();" >Add Call Detail</asp:LinkButton>

    <asp:HiddenField ID="HiddenField2" Value="" runat="server" />

    <iframe id= "iframeAddDetail"  width="910px" height="500px" src = ""></iframe> 

Thanks 

Yashvant