ModalPopupExtender not hiding

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ASP

Ajax

I have a modal popup in asp.net. when i'm click delete button it show popup. but when i click other buttons popup window show again and closing. Please help to slove this.

protected void btnDeleteOK_Click(object sender, EventArgs e)
   {
       ModalPopupExtender1.Show();
   }

<ajaxtoolkit:ModalPopupExtender ID="mpdelete"  runat="server" CancelControlID="btnDeleteCancel" DropShadow="true" TargetControlID="HiddenField2" PopupControlID="DeleteDiv"  BackgroundCssClass="confirmBackground">
 </ajaxtoolkit:ModalPopupExtender>
              <asp:HiddenField ID="HiddenField2" runat="server" />
              <div  runat="server" id="DeleteDiv" class="panel panel-success"  style=" background-color:#E6F3E6; width: 250px; padding-top: 10px; border-style:solid;   border-color:darkgreen;border-top-left-radius: 6px; border-top-right-radius: 6px; padding: 6px;border-radius: 4px;">

                           <div class="row">
                              <div class="col-sm-12">
                                    <asp:Label ID="Label1"  runat="server" Text="Are You Sure, You Want to Delete This Record?" Font-Size="Small" ForeColor="DarkGreen"></asp:Label>
                                    </div>
                          </div>
                             <div class="row">
                              <div class="col-sm-12">
                                   <div class="col-sm-2">
                                       </div>

                                   <div class="col-sm-8">
                                          <asp:Button ID="btnDeleteOK" runat="server" Width="55px" Height="28px" Text="Ok" CssClass="btn btn-success button"  Font-Size="Smaller" OnClick="btnDeleteOK_Click"/>
                                       <asp:Button ID="btnDeleteCancel" Width="55px" Height="28px" runat="server" Text="Cancel" CssClass="btn btn-success button" Font-Size="Smaller"/>

                                  </div>
                                    <div class="col-sm-2">
                                       </div>
                              </div>
                          </div>

                       </div>