panel or div used for modalpopup is appearing when clicking button.

Hi,

i have modalpop up as below

<!-- Extender and its contorls -->
    <asp:Label ID="hdlblRoleId" runat="server" Visible="false"></asp:Label>
    <asp:Button ID="btnShowPopup" style="display:none" runat="server" />
    <ajax:ModalPopupExtender ID="modalBranch" runat="server" TargetControlID="btnShowPopup"   
                            Enabled="True"  BehaviorID="MPE" BackgroundCssClass="modalBackground" CancelControlID="btnClose"
                            PopupControlID="searchpopup">
    </ajax:ModalPopupExtender>
    <!-- Extender end here -->

and this is pop up

  <div id="searchpopup">
                <div id="searchpopuptopclose">

..

..

</div>

</div>

After clicking button, i have script to show successful added with below

string msg = "Branch added successfully.";
ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('" + msg + "');window.location ='AddBranch.aspx';", true);

I have button and when it clicks , save the data into db and show in grid. when i click link on the grid, pop up will come and let user edit.

when i clicked add button to save date,  I can see message which is showing successfully added and also pop up div is showing at the  bottom of the page where it place  on the page.

please advice.