Nested GridView In ASP.NET Using C# (Parent And Child Gridview)

I'm trying to add a nested gridview to a parent gridview.

The code working correctly but I have a problem when I reload the page using

protected void btreload_Click(object sender, ImageClickEventArgs e)
{        
    Response.Redirect(HttpContext.Current.Request.Url.AbsoluteUri);
    BindData();
}

Because the button in parent gridview of getting nested gridview don't longer open the nested gridview...

This is my code gv_parent_RowDataBound

if (e.Row.DataItem != null)
{
    GridView gv_Child = (GridView)e.Row.FindControl("gv_Child");

    if (gv_Child != null)
    {
        string customerId = gv_parent.DataKeys[e.Row.RowIndex].Value.ToString();
        DataSet ds = new DataSet();

        using (MySqlConnection myConnectionString =
            new MySqlConnection(ConfigurationManager.ConnectionStrings["cn"].ConnectionString))
        {
            using (MySqlCommand cmd =
                new MySqlCommand("SP_CHILD", myConnectionString))
            {
                cmd.Connection.Open();
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("tun", Request.QueryString["un"]);
                cmd.Parameters.AddWithValue("tmo", Request.QueryString["mo"]);
                cmd.Parameters.AddWithValue("tcustomerId", customerId);

                MySqlDataAdapter adapter = new MySqlDataAdapter();
                adapter = new MySqlDataAdapter(cmd);
                adapter.Fill(ds);
                cmd.Connection.Close();

                gv_Child.DataSource = ds.Tables[0];
                gv_Child.DataBind();
            }
        }
    }