How to deal with exception handling? this code shows error SqlDataReader rs = cmd.ExecuteReader();

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;
using System.Collections;

namespace AutoStatus
{

    
    public partial class Form1 : Form
    {
        
        bool AppAdded=false;
        SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\v11.0;");

        public Form1()
        {
            InitializeComponent();
            
            getapp();
        }

        private void getapp()
        {
            SqlCommand cmd = new SqlCommand("SELECT app FROM mytable ", conn);
            this.conn.Open();
            SqlDataReader rs = cmd.ExecuteReader();
            ArrayList Authors = new ArrayList();

            while (rs.Read())
            {
                Authors.Add(new AddValue(rs.GetString(0)));

            }
            rs.Close();
            this.conn.Close();

            this.comboBox2.DataSource = Authors;
            this.comboBox2.DisplayMember = "app";

            AppAdded = true;
        }



        private void getcomponents()
        {
            this.comboBox3.Items.Clear();
           SqlCommand cmd = new SqlCommand("SELECT components FROM mytable WHERE app =" + this.comboBox2.SelectedValue, conn);
            this.conn.Open();
            SqlDataReader rs = cmd.ExecuteReader();
            while (rs.Read())
            {
                this.comboBox3.Items.Add(rs.GetString(2));
            }
            rs.Close();
            this.conn.Close();

        }

        /*private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Items.Add(new ComboBoxItem("item1 text", "item1 value"));
            MessageBox.Show(((ComboBoxItem)comboBox1.Items[0]).Value);
            /*
            Dictionary comboSource = new Dictionary<string,>();
            comboSource.Add("1", "Sunday");
            comboBox2.Items.Add("weekdays");
            comboBox2.Items.Add("year");*/
        

        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (this.AppAdded)
                getcomponents();
        }
    }

   

    public class AddValue
    {
        private string m_Display;
        
        public AddValue(string Display)
        {
            m_Display = Display;
            
        }
        public string Display
        {
            get { return m_Display; }
        }
      
    }
}