ModalPopupExtender doesn't recognize TargetControlID in Gridview

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ASP.NET

Hi all.

I am getting an error when i call the Link_button id which is inside GridView Item template.

then i change TargetControlI to hidden button as following:

<
asp:HiddenField ID="hfHidden" runat="server" />
                           <ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender" runat="server" TargetControlID="hfHidden"
       PopupControlID="Panel1" BackgroundCssClass="modalBackground" OkControlID="OkButton"
       >

       <asp:GridView ID="GridView1" runat="server"  AutoGenerateColumns="False" 
                            dir="ltr" CellPadding="4" ForeColor="#333333" GridLines="None" 
                            onselectedindexchanged="GridView1_SelectedIndexChanged" Width="220px" 
                            AllowSorting="True" DataSourceID="SqlDataSource1" Font-Names="B Yekan" 
                            DataKeyNames="MessageId" onrowcommand="GridView1_RowCommand" 
                            onrowdatabound="GridView1_RowDataBound">
                            <AlternatingRowStyle BackColor="White" />
                                    <Columns>
<asp:TemplateField HeaderText="show"><EditItemTemplate>
                                                            
</EditItemTemplate>
<ItemStyle Width="40%" CssClass="Col1" HorizontalAlign="Center" 
        VerticalAlign="Middle"></ItemStyle>
<ItemTemplate>

                                <asp:LinkButton ID="LinkButton_Subject" runat="server" CommandArgument="<%# Container.DataItemIndex %>"
                                    OnClick="LinkButton_Subject_Click" Text="show" CommandName="ViewComments"></asp:LinkButton>
                                <asp:Label ID="Label_MessageID" runat="server" Text='<%# Eval("MessageID") %>' Visible="False"></asp:Label>

                          
</ItemTemplate>
</asp:TemplateField>

and in code behind :

  protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
        {
            switch (e.CommandName)  
            {
                case "ViewComments":
                    ModalPopupExtender.Show();
                    break;
            } 
        }

and 

protected void LinkButton_Subject_Click(object sender, EventArgs e)
        {
            
            ModalPopupExtender.Show();
         
  
        
        }

but when click on link button , don't show popup panel

thanks a lot.