ModalPopupExtender in asp.net

 <asp:Button ID="btnTest" runat="server" Text="Click Here" />
       <ajax:ModalPopupExtender ID="mpopResArea" runat="server" OkControlID="imgClose_resArea"
     PopupControlID="pnlResArea" BackgroundCssClass="modalBackground" TargetControlID="btnTest">     
    </ajax:ModalPopupExtender>
   <asp:Panel ID="pnlResArea" runat="server" style="display:none" BackColor="#F9F6F4" Height="350px" Width="380px" >
     <div class="PopUpheader">
                    <span id="Span1" class="msg">Select Restricted Area</span>
                    <span style="float:right">
                    <asp:ImageButton ID="imgClose_resArea" AlternateText="Close" runat="server" ImageUrl="~/images/close.gif" /></span>
                </div>          
                <table width="85%" class="cssform">
                <tr>    
                  <td>
                    Select Area :
                  </td>            
                   <td>
                     <asp:DropDownList ID="ddlResArea" runat="server" AutoPostBack="true" AppendDataBoundItems="true" onselectedindexchanged="ddlResArea_SelectedIndexChanged">
                       <asp:ListItem Text="--Select Area--" Value="0"></asp:ListItem>
                     </asp:DropDownList>           
                  </td>
                </tr>
                </table>
                <br />
                <center>
                     <dx:ASPxGridView ID="grvResarea_aspx" AutoGenerateColumns="False" KeyFieldName="Id;RestrictedAreaId"  runat="server"
                            OnRowCommand="grvResarea_aspx_RowCommand">
                            <Columns>               
                            <dx:GridViewDataTextColumn Caption="ID" FieldName="Id" Visible="false"></dx:GridViewDataTextColumn>                              
                            <dx:GridViewDataColumn Caption="Select">
                             <DataItemTemplate>
                               <asp:LinkButton ID="LinkButton1" runat="server" Text="Select" CommandName="Select" CommandArgument='<%# Eval("Id") %>'></asp:LinkButton>                              
                              </DataItemTemplate>
                            </dx:GridViewDataColumn>                      
                              <dx:GridViewDataTextColumn Caption="Restricted AreaID" VisibleIndex="2" FieldName="RestrictedAreaId"></dx:GridViewDataTextColumn>
                               <dx:GridViewDataTextColumn Caption="Site Name" VisibleIndex="3" FieldName="SiteName"></dx:GridViewDataTextColumn>
                            </Columns>                                                        
                 </dx:ASPxGridView>
                </center>
      </asp:Panel>