System.NullReferenceException:末将对象引用设置到对象实例。这该怎么改？ 问题在内容里

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.Configuration;

namespace chaohsi
{
public partial class LoginForm : Form
{
//定义连接字符串变量,从配置文件中读取连接字符串
private string conString = ConfigurationManager.ConnectionStrings["chaoshi"].ConnectionString.ToString();
public LoginForm()
{
InitializeComponent();
}
///


/// 登录处理程序
///
///
///
private void button1_Click(object sender, EventArgs e)
{
//（1）获取输入数据
string userid = this.txtUserID.Text.Trim();
string pwd = this.txtPwd.Text.Trim();
//(2)建立数据库连接对象
//连接字符串
string conString = @"Data Source=LAPTOP-0KJBJLJG\MSSQLSERVER01;Initial Catalog=db_chaoshi;Integrated Security=true";
//构造SQL命令
string strSQL = string.Format("select * from [tb_User] where UserID='{0}' and UserPwd='{1}'", userid, pwd);
using (SqlConnection con = new SqlConnection(conString))
{
//建立命令对象
SqlCommand cmd = new SqlCommand(strSQL, con);
//打开数据库连接
con.Open();
//执行命令,返回结果集
//object obj = cmd.ExecuteScalar();
SqlDataReader sdr = cmd.ExecuteReader();
//根据结果处理
if (sdr.HasRows == false) //没有指定用户
{
MessageBox.Show("用户名或密码错误！", "用户登录");
this.txtUserID.Clear();
this.txtPwd.Clear();
this.txtUserID.Focus();
}
else
{
MessageBox.Show("登录成功！", "用户登录");
//读取用户名，保存
sdr.Read();
//将用户姓名保存
Users.userName = Convert.ToString(sdr["UserName"]);
//记录登录窗体的结果
this.DialogResult = DialogResult.OK;
}
}
}
}
class Users
{
public static string userName { set; get; }
}
}

private string conString = ConfigurationManager.ConnectionStrings["chaoshi"].ConnectionString.ToString();这句话说引发异常，System.NullReferenceException:末将对象引用设置到对象实例

你的chaoshi 要么写错了 要么就空了