对象引用未设置为对象的实例(计算器项目错误)

好的，我很确定我的代码都是正确的，所以我不确定为什么我得到这个错误。:P这是我的代码——>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;
namespace Calculator
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());  //This is where I am getting the error at
        }
    }
}

在我的program.cs文件中。这是我的form。cs文件。——>

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 Calculator
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void btn0_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn0.Text;
        }
        private void btn1_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn1.Text;
        }
        private void btn2_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn2.Text;
        }
        private void btn3_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn3.Text;
        }
        private void btn4_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn4.Text;
        }
        private void btn5_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn5.Text;
        }
        private void btn6_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn6.Text;
        }
        private void btn7_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn7.Text;
        }
        private void btn8_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn8.Text;
        }
        private void btn9_Click(object sender, EventArgs e)
        {
            txtbox.Text += btn9.Text;
        }
        private void btnPoint_Click(object sender, EventArgs e)
        {
            if (txtbox.Text == "0")
            {
                txtbox.Text = "0.";
            }
            else
            {
                txtbox.Text += btnPoint.Text;
            }
        }
        private void btnPlus_Click(object sender, EventArgs e)
        {
            //Add plus code here
        }
        private void btnMinus_Click(object sender, EventArgs e)
        {
            //Add minus code here
        }
        private void btnTimes_Click(object sender, EventArgs e)
        {
            //Add times code here
        }
        private void btnDivide_Click(object sender, EventArgs e)
        {
            //Add divide code here
        }
        private void btnEqual_Click(object sender, EventArgs e)
        {
            //Add equal code here
        }
        private void btnClear_Click(object sender, EventArgs e)
        {
            txtbox.Clear();
        }
    }
}

当我尝试先输入一个数字，然后输入一个小数时发生错误。例如，我按"2"，然后按"。"。会出现错误"对象引用未设置为对象的实例"。帮助是感激的，谢谢。抱歉，如果这是一个愚蠢的错误。: P