How to Editing the gridview using modelpopup extender.But the button is outside the gridview in asp.net?

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ASP.NET

protected void popuplink_Click(object sender, EventArgs e)
{
try
{

foreach (GridViewRow i in gvuser.Rows)
{


RadioButton rb = (RadioButton)i.FindControl("Radselect");
if (rb.Checked && rb != null)
{

txtmodusername.Text = i.Cells[2].Text;
txtfullname.Text = i.Cells[3].Text;
txtmodemail.Text = i.Cells[4].Text;
ddlmodpermission.SelectedItem.Text = i.Cells[5].Text;
Checkbox3.Visible = i.Cells[6].Visible;


this.Btn_Modify_ModalPopupExtender.Show();
}
}
}

<asp:Button ID="popuplink"
                                runat="server" Text="Modify" OnClick="popuplink_Click" /><cc1:ModalPopupExtender
                                    ID="Btn_Modify_ModalPopupExtender" runat="server" TargetControlID="Btntest" CancelControlID="popupclose"
                                    PopupControlID="Panel2" Enabled="True" DynamicServicePath="" BackgroundCssClass="ModalPopupBG">
                                </cc1:ModalPopupExtender>

When i am debuging,values assign to textbox controls.but popup does not show.