gridview c#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

public partial class Pregled : System.Web.UI.Page
{
    List<string> lista;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ispolniCustomers();
        }


    }

    protected void ispolniCustomers()
    {
        SqlConnection konekcija = new SqlConnection();
        konekcija.ConnectionString = ConfigurationManager.ConnectionStrings["mojaKonekcija"].ConnectionString;
        string sqlString = "SELECT * FROM Customers";
        SqlCommand komanda = new SqlCommand(sqlString, konekcija);
        SqlDataAdapter adapter = new SqlDataAdapter(komanda);
        DataSet ds = new DataSet();

        try
        {
            konekcija.Open();
            adapter.Fill(ds);
            gvCustomers.DataSource = ds;
            gvCustomers.DataBind();
            ViewState["dataset"] = ds;


        }
        catch(Exception err)
        {
            lblPoraka.Text = err.Message;
        }
        finally
        {
            konekcija.Close();
        }

    }



    protected void gvCustomers_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        gvCustomers.SelectedIndex = -1;
        gvCustomers.PageIndex = e.NewPageIndex;
        DataSet ds = (DataSet)ViewState["dataset"];
        gvCustomers.DataSource = ds;
        gvCustomers.DataBind();
        gvOrders.Visible = false;
    }


    protected void ispolniOrders()
    {
        SqlConnection konekcija = new SqlConnection();
        konekcija.ConnectionString = ConfigurationManager.ConnectionStrings["mojaKonekcija"].ConnectionString;
        string sqlString = "SELECT Customers.CompanyName, Products.ProductName, Products.UnitPrice, Orders.OrderID, OrderDate FROM Customers INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID INNER JOIN [Order Details] ON Orders.OrderID = [Order Details].OrderID INNER JOIN Products ON [Order Details].ProductID = Products.ProductID WHERE Customers.CustomerID=@CustomerID";
        SqlCommand komanda = new SqlCommand(sqlString, konekcija);
        komanda.Parameters.AddWithValue("@CustomerID", gvCustomers.DataKeys[gvCustomers.SelectedIndex].Value);
        SqlDataAdapter adapter = new SqlDataAdapter(komanda);
        DataSet ds = new DataSet();

        try
        {
            konekcija.Open();
            adapter.Fill(ds);
            gvOrders.DataSource = ds;
            gvOrders.DataBind();
            gvOrders.Visible = true;

        }
        catch (Exception err)
        {
            lblPoraka.Text = err.Message;
        }
        finally
        {
            konekcija.Close();
        }
    }

    protected void gvCustomers_SelectedIndexChanged(object sender, EventArgs e)
    {
        ispolniOrders();
    }

    protected void btnPayment_Click(object sender, EventArgs e)
    {
        if (gvOrders.SelectedIndex != -1)
        {
            string produkt = gvOrders.SelectedRow.Cells[2].Text;
            if (Session["smetka"] == null)
            {
                lista = new List<string>();
                lista.Add(produkt);
                Session["smetka"] = lista;
            }
            else
            {
                lista = (List<string>)Session["smetka"];
                lista.Add(produkt);
                Session["smetka"] = lista;
            }
        }

        Response.Redirect("Payment.aspx");
    }
}