C#通过配置文件连接SQL数据库报错！System.NullReferenceException: 未将对象引用设置到对象的实例

App.config文件如下：
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="conStr" connectionString="Data Source=DELL-PC;Initial Catalog=mydb2;Integrated Security=True"
      providerName="System.Data.SqlClient" />
  </connectionStrings>
</configuration>
 
connTest.cs文件如下：
using System;
using System.ComponentModel;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Configuration;
using System.Data.SqlClient;
using System.Windows.Forms;
using System.Drawing;
namespace MyProject
{
    public partial class connTest:Form
    {
        public connTest()
        {
            InitializeComponent();
        }
 
        public string GetConnectionString()
        {
           string connStr = ConfigurationManager.ConnectionStrings["connStr"].ConnectionString; // 这句报错
           if (connStr!= null)
            {
 
                return connStr;
            }
            else
            {
                return null ;
            }
        }
 
        private void connTest_Load(object sender, EventArgs e)
        {
 
            SqlConnection conn = new SqlConnection(GetConnectionString());
                try
                {
                    conn.Open();
                    if (conn.State == ConnectionState.Open)
                    {
                        label1.Text = "该连接字符串为：\r\n";
                        label1.Text += GetConnectionString() + "\r\n";
                        label1.Text += "数据库连接成功";
                    }
                }
                catch (Exception ex)
                {
                    label1.Text = ex.Message;
                }
                finally
                {
                    conn.Close();
                }
            }
        }
    }
急救啊