C#未将对象引用设置到对象的实例。

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;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public windows1 w1;
        public windows2 w2;

        private void Form1_Load(object sender, EventArgs e)
        {
            w1 = new windows1();
            w2 = new windows2();

        }

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            w1.Show();
            gpbWindows.Controls.Clear();
            gpbWindows.Controls.Add(w1);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            w2.Show();
            gpbWindows.Controls.Clear();
            gpbWindows.Controls.Add(w2);
        }

        private void button4_Click(object sender, EventArgs e)
        {

        }
    }
}
 

请问是那行代码报“”未将对象引用设置到对象的实例“”？