ช่วยด้วยครับ มันขึ้น Fill: SelectCommand.Connection property has not been initialized.

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 BusOnline
{
    public partial class Form3 : Form
    {
        DataSet ds; SqlCommand cmd; SqlDataAdapter adp;
        SqlConnection conn; 
        public Form3()
        {
            InitializeComponent();
        }

        private void Form3_Load(object sender, EventArgs e)
        {
            OpenConnectionDatabase();
            conn.Close();
        }
               
        private void OpenConnectionDatabase()
        {
            string conStr = @"data source =.\sqlexpress;attachDbFilename = D:\Project\BusDB\db.mdf;";
            conStr += "Integrated Security = True;";
            conn = new SqlConnection(conStr);
            open_conn();
            clearData();
        }

        private void clearData()
        {
            textBox1.Text = ""; textBox2.Text = "";
        }

        private void open_conn()
        {
            if (conn.State == ConnectionState.Closed) conn.Open();
        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if (textBox1.Text.Trim() == "") { return; }
            if (e.KeyCode == Keys.Enter)
            {
                string sql = "select cuscode from customer where cuscode = @cusc";
                cmd = new SqlCommand(sql, conn);
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = sql;
                cmd.Parameters.Clear();
                cmd.Parameters.Add("@cusc", SqlDbType.NChar).Value = textBox1.Text.Trim();
                adp = new SqlDataAdapter(cmd);
                ds = new DataSet();
                int p = adp.Fill(ds,"cus");
                if (p > 0) 
                {
                    textBox2.Text = ds.Tables["cus"].Rows[0]["cusc"].ToString();
                    textBox1.Focus();
                }
                else
                {
                    MessageBox.Show("Identify not found"); clearData();                    
                }
            }            
        }  
       
        private void button2_Click(object sender, EventArgs e)
        {
            this.Hide();
            FormMenuMain f = new FormMenuMain();
            f.Show();
        }

        private void button1_Click_1(object sender, EventArgs e)
        {
            if (textBox1.Text == textBox2.Text && textBox1.Text.Length > 0)
            {
                this.Hide();
                FormSub2 f = new FormSub2();
                f.Show();
            }
            else
            {
                MessageBox.Show("Identify not compare");
                clearData();
            }
        }              
    }
}