System.NullReferenceException was unhandled ?

Here's my code

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

namespace Store_Goods_List
{
    public partial class Form1 : Form
    {
        SqlConnection cn = new SqlConnection(@"Data Source=(LocalDB)\v11.0;AttachDbFilename=c:\users\miki\documents\visual studio 2012\Projects\Store Goods List\Store Goods List\Database1.mdf;Integrated Security=True");
        SqlCommand cmd = new SqlCommand();
        SqlDataReader dr;
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            cmd.Connection = cn;
            loadlist();
        }

        private void label3_Click(object sender, EventArgs e)
        {
            if (txtnum.Text != "" & txtname.Text != "" & txtstock.Text != "" & txtprice.Text!="")
            {
                cn.Open();
                cmd.CommandText = "Insert Info (product id,name,stock,price) values ('"+txtnum.Text+"','"+txtname.Text+"','"+txtstock.Text+"','"+txtprice.Text+"')";
                cmd.ExecuteNonQuery();
                cmd.Clone();
                MessageBox.Show("Record inserted!","Programming");
                cn.Close();
                txtnum.Text = "";
                txtname.Text = "";
                txtstock.Text = "";
                txtprice.Text = "";
                loadlist();
            }
        }
        private void loadlist()
        {
            listBox1.Items.Clear();
            listBox2.Items.Clear();
            listBox3.Items.Clear();
            listBox4.Items.Clear();
            cn.Open();
            cmd.CommandText = "select ' from info";
            dr = cmd.ExecuteReader();
            if (dr.HasRows)
            {
                while (dr.Read())
                {
                    listBox1.Items.Add(dr[0].ToString());
                    listBox2.Items.Add(dr[1].ToString());
                    listBox3.Items.Add(dr[2].ToString());
                    listBox4.Items.Add(dr[3].ToString());
                }
            }
            cn.Close();
        }

        private void listBox4_SelectedIndexChanged(object sender, EventArgs e)
        {
            ListBox l = sender as ListBox;
            if(l.SelectedIndex!= -1)
            {
                listBox1.SelectedIndex = l.SelectedIndex;
                listBox2.SelectedIndex = l.SelectedIndex;
                listBox3.SelectedIndex = l.SelectedIndex;
                listBox4.SelectedIndex = l.SelectedIndex;
            }
        }
    }
}