have problem with system.NullReferenceException

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace WindowsFirstsample
{
    public partial class Form1 : Form
    {
        private SqlConnection cn;
       
        private SqlDataAdapter da;
        private SqlCommand cmd;
        private DataSet ds;
       
        private string strcon;
        private string strq;
        private int icounter;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            strcon="Data Source=HP-PC//SQLEXPRESS;Initial Catalog=school;Integrated security=true; ";
            
            cn = new SqlConnection(strcon);
            
            strq = "select * from student";
            cn.Open();
            da = new SqlDataAdapter(strq,cn);
           
            ds = new DataSet();
          
            da.Fill(ds);
            textBox1.Text = ds.Tables[0].Rows[0]["id"].ToString();
            textBox2.Text = ds.Tables[0].Rows[0]["name"].ToString();
        }

       

        private void button1_Click(object sender, EventArgs e)
        {
            icounter = 0;
            textBox1.Text = ds.Tables[0].Rows[0]["id"].ToString();
            textBox2.Text = ds.Tables[0].Rows[0]["name"].ToString();

        }

        private void button2_Click(object sender, EventArgs e)
        {
            int irowcount = ds.Tables[0].Rows.Count - 1;
            if (icounter < irowcount)
            {
                icounter++;
                textBox1.Text = ds.Tables[0].Rows[icounter]["id"].ToString();
                textBox2.Text = ds.Tables[0].Rows[icounter]["name"].ToString();
            }
            else
            {
                MessageBox.Show("you are already on the last record");
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            if (icounter > 0)
            {
                icounter--;
                textBox1.Text = ds.Tables[0].Rows[icounter]["id"].ToString();
                textBox2.Text = ds.Tables[0].Rows[icounter]["name"].ToString();
            }
            else
            {
                MessageBox.Show("you are already on the first record");
            }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            icounter = ds.Tables[0].Rows.Count - 1;
            textBox1.Text = ds.Tables[0].Rows[icounter]["id"].ToString();
            textBox2.Text = ds.Tables[0].Rows[icounter]["name"].ToString();
        }
    }
}