在Visual Studio 2010中完成C#作业时遇到问题

好的，所以我一直在尝试获得此任务中所有员工的总数、工人数量、总工资和平均工资的摘要。但是，当我在程序运行时单击程序中的"摘要按钮"时，我不断收到错误消息。"EX0406 中发生了类型为'System.NullReferenceException'的未处理异常.exe其他信息：对象引用未设置为对象的实例。

这是我收到的错误消息。我不太确定该怎么做。

看到我卡住了，我的代码不完整，但我希望得到有关如何显示摘要的任何帮助。我是编程新手。 如果我需要更具体，请告诉我。

这是我所拥有的：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace EX0406
{
    public partial class PieceworkForm : Form
    {    //class variables 
        private string name;
        private int pieces;
        private string Average_Pay_Per_PERSON;
        private decimal total_Pay; 

        private List<MyData> MyList = new List<MyData>();
        public PieceworkForm()
        {
            InitializeComponent();
        }
        private void calculateButton_Click(object sender, EventArgs e)
        {
            //does the calculations 
            string name = (EmployeeNameTextBox).Text;
            int pieces = Convert.ToInt32(NumOfPiecesText.Text);
            if (name == "")
               MessageBox.Show("You must enter a name.");
            else if (pieces < 1 || pieces > 799)
               MessageBox.Show("Enter a number between 1 and 799.");
            else
            {
               MyData data = new MyData(name, pieces);
               MyList.Add(data);
               MessageBox.Show(string.Format("Amount Earned: {0:C}", data.Paid));
            }
        } 
        private void summaryButton_Click(object sender, EventArgs e)
        {       
            //Display info in msg box
            MyData data = new MyData(name, pieces);
            MyList.Add(data);
            try
            {
                string summaryString = ""
                                     + pieces.ToString()
                                     + "'n'n" + "Total pay:"
                                     + total_Pay.ToString("C")
                                     + "'n'n" + "Average pay per person:"
                                     + Average_Pay_Per_PERSON.ToString();
                MessageBox.Show(summaryString, "Summary:");
            }
            finally
            { 
                MessageBox.Show(SummaryOutPut.Text);  
            }
        }
        private void clearButton_Click(object sender, EventArgs e)
        {   //clears the employee and number of pieces text box
            EmployeeNameTextBox.Clear();
            NumOfPiecesText.Clear();
        }
        private void clearAllButton_Click(object sender, EventArgs e)
        {
            EmployeeNameTextBox.Clear();
            NumOfPiecesText.Clear();
            SummaryOutPut.Clear(); 
        }
        private void exitButton_Click(object sender, EventArgs e)
        {   //closes the program when user clicks exit
            this.Close(); 
        }
        private void nameTextBox_TextChanged(object sender, EventArgs e)
        {
        }

        public class MyData
        {
            public MyData(string name, int pieces)
            {
                //if else statements 
                Name = name;
                Pieces = pieces;
                if (Pieces < 200) //if pieces are less than 200, multiply by .50
                    Paid = Pieces * .50;
                else if (Pieces < 400)  //mulitply by .55 if less than 400
                    Paid = Pieces * .55;
                else if (Pieces < 600) // multiply by .60 if less than 600
                    Paid = Pieces * .60;
                else  // multiply by .65 if everything else doesnt apply 
                    Paid = Pieces * .65;
            }
            public string Name;
            public int Pieces;
            public double Paid;
        }
        private void SummaryOutPut_TextChanged(object sender, EventArgs e)
        {
        }
        private void fileToolStripMenuItem_Click(object sender, EventArgs e)
        {
        }
        private void helpToolStripMenuItem_Click_1(object sender, EventArgs e)
        {
        }
        /* the following lines of code are menu items*/
        private void aboutToolStripMenuItem_Click_1(object sender, EventArgs e)
        {   //MSG box that displays name of lab and my name when clicking about and help item.
            MessageBox.Show("Lab 5 by J Soto"); 
        }
        private void exitToolStripMenuItem_Click_1(object sender, EventArgs e)
        {    //exit menu item that closes the windows form. 
            this.Close(); 
        }
        private void clearToolStripMenuItem_Click(object sender, EventArgs e)
        {    //menu item that clears the employee name and # of pieces text box.
            EmployeeNameTextBox.Clear();
            NumOfPiecesText.Clear();
        }
        private void clearAllToolStripMenuItem_Click(object sender, EventArgs e)
        {    //menu item to clear all the text boxes.
            EmployeeNameTextBox.Clear();
            NumOfPiecesText.Clear();
            SummaryOutPut.Clear(); 
        }
        private void fontToolStripMenuItem_Click(object sender, EventArgs e)
        {
        }
        private void colorToolStripMenuItem_Click(object sender, EventArgs e)
        {
        }
        private void calculatePayToolStripMenuItem_Click(object sender, EventArgs e)
        {  //does the calculations 
            string name = (EmployeeNameTextBox).Text;
            int pieces = Convert.ToInt32(NumOfPiecesText.Text);
            if (name == "")
                MessageBox.Show("You must enter a name.");
            else if (pieces < 1 || pieces > 799)
                MessageBox.Show("Enter a number between 1 and 799.");
            else
            {
                MyData data = new MyData(name, pieces);
                MyList.Add(data);
                MessageBox.Show(string.Format("Amount Earned: {0:C}", data.Paid));
            }
        } 
        private void summaryToolStripMenuItem_Click(object sender, EventArgs e)
        {
        }
    }
}