如何修复列表框中的此nullreferenceexception

很抱歉再次问这个问题，但我真的不知道如何调试它
调试器在以下行中为我提供nullreferenceexception：if（listBox1.SelectedItem.ToString（）=="鸡肉$15"）我想它给我nullreferenceexception的原因是因为listbox1为null，所以我认为我必须初始化它。但怎么初始化呢？我不知道如何初始化列表框。

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;
namespace lab6
{
    public partial class Form1 : Form
{
    double total = 0;
    int x = 0;
    string ord = "";
    public Form1()
    {
        InitializeComponent();
    }
    private void toolStripMenuItem1_Click(object sender, EventArgs e)
    {
    }
    private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
    }
    private void Form1_Load(object sender, EventArgs e)
    {
    }
    private void menuStrip1_ItemClicked(object sender, ToolStripItemClickedEventArgs e)
    {
    }
    private void editToolStripMenuItem_Click(object sender, EventArgs e)
    {
    }
    private void placeToolStripMenuItem_Click(object sender, EventArgs e)
    {
        checkBox1.Checked = false;
        radioButton1.Checked = false;
        radioButton2.Checked = false;
        radioButton3.Checked = false;
        radioButton4.Checked = false;
        switch (checkBox1.Checked)
        {
         case true:
           total += 1;
            ord += "Water'n";
             break;
        }
        if (comboBox1.Text == "Extra Meat")
        {
            total += 1;
            ord += ord + "Extra Meat'n";
        }
        if (comboBox1.Text == "Extra Rice")
        {
            total += 1;
            ord += "Extra Rice'n";
        }
        if (comboBox1.Text == "Extra Veggies")
        {
            total += 1;
            ord += "Extra Veggies'n";
        }


        if (listBox1.SelectedItem.ToString() == "Chicken $15")
        {
            total += 15;
            ord += " Chicken'n";
        }
        else { }

            if (listBox1.SelectedItem.ToString() == "Pizza $8")  //< my pathetic attempt to figure it out with intelisense
            {
                total += 8;
                ord += "Pizza'n";
            }
            else
            { 
            }
        if (listBox1.SelectedItem.ToString() == "Spaghetti $12")//< my pathetic attempt to figure it out with intelisense
        {
            total += 12;
            ord += " Spaghetti'n";
        }
        else { }
        if (listBox1.SelectedItem.ToString() == "Fries $8")
        {
            total += 8;
            ord += " Fries'n";
        }
        else { }
        if (listBox1.SelectedItem.ToString() == "Burger $10")
        {
            total += 10;
            ord += " Burger'n";
        }
        else { }
        //radiobutton
        if (radioButton1.Checked)
        { 
            total+=5;
            ord += "Pineapple Juice'n";
        }
        if (radioButton2.Checked)
        {
            total+=6;
            ord += "Mango Juice'n";
        }
        if (radioButton3.Checked)
        {
            total+=7;
            ord += "Apple Juice'n";
        }
        if (radioButton4.Checked)
        {
            total+=8;
            ord += "Orange Juice'n";
        }
        MessageBox.Show("Order Done");
        listBox1.SelectedItems.Clear();
    }
    private void clearToolStripMenuItem_Click(object sender, EventArgs e)
    {
        ord = "";
        total = 0;
    }
    private void radioButton1_CheckedChanged(object sender, EventArgs e)
    {
    }
    private void label3_Click(object sender, EventArgs e)
    {
    }
    private void radioButton2_CheckedChanged(object sender, EventArgs e)
    {
    }
    private void radioButton4_CheckedChanged(object sender, EventArgs e)
    {
    }
    private void radioButton3_CheckedChanged(object sender, EventArgs e)
    {
    }
    private void checkBox1_CheckedChanged(object sender, EventArgs e)
    {
    }
    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
    }
    private void label4_Click(object sender, EventArgs e)
    {
    }
    private void displayToolStripMenuItem_Click(object sender, EventArgs e)
    {
        MessageBox.Show("Order: " + ord+"'nTotal: "+total);

    }
    private void exitToolStripMenuItem_Click(object sender, EventArgs e)
    {
        this.Close();
    }
}

}

if (listBox1.SelectedItem != null)
{
    // code here
}