一个从列表框中删除选中项的问题，求解决
我想实现这个程序
点删除后出现这个
求大神帮我看看代码

public partial class Form1 : Form
    {

        
        public Form1()
        {
            InitializeComponent();
        }
        static List<user> user1 = new List<user>();
        private void btn_add_Click(object sender, EventArgs e)//添加按钮
        {
           
            listBox1.Items.Add(txb_user.Text);
            user u = new user();
            u.userName = txb_user.Text;
            u.userpassword = txb_password.Text;
            if (rdb_admin.Checked)
            {
                u.select = rdb_admin.Text;
            }
            if (rdb_read.Checked)
            {
                u.select = rdb_read.Text;
            }
            if (rdb_readwrite.Checked)
            {
                u.select = rdb_readwrite.Text;

            }
            if (rdb_write.Checked)
            {
                u.select = rdb_write.Text;
            } 
 
            user1.Add(u);
            //清屏
            txb_user.Clear();
            txb_password.Clear();
            text_rpassword.Clear();
            if (rdb_admin.Checked)
                rdb_admin.Checked = false;
            if (rdb_read.Checked)
                rdb_read.Checked = false;
            if (rdb_readwrite.Checked)
                rdb_readwrite.Checked = false;
            if (rdb_write.Checked)
                rdb_write.Checked = false;
            
        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)//当选择改变时
        {
            int n = listBox1.SelectedIndex;//获取列表框的当前集合
            
            
                txb_user.Text = listBox1.Items[n].ToString();
                for (int i = 0; i < user1.Count; i++)
                {
                    if (txb_user.Text == user1[i].userName)
                    {
                        txb_password.Text = user1[i].userpassword;
                        text_rpassword.Text = user1[i].userpassword; 
 
                        string quanxian = user1[i].select;
                        switch (quanxian)
                        {
                            case "管理员":
                                rdb_admin.Checked = true;
                                break;
                            case "只读用户":
                                rdb_read.Checked = true;
                                break;
                            case "只写用户":
                                rdb_write.Checked = true;
                                break;
                            case "读写用户":
                                rdb_readwrite.Checked = true;
                                break;
                        } 
 
                    }
                
            }
        }
        
        private void Form1_Load(object sender, EventArgs e)
        {

        }

        

        private void btn_alter_Click(object sender, EventArgs e)//修改还没做
        {

            int n = listBox1.SelectedIndex;//获取列表框的当前集合
            txb_user.Text = listBox1.Items[n].ToString();
            
            
        }
        class user//用于保存集合中TextBox的内容
        {
          public  string userName;
           public string userpassword;
           public string select;
        }

        private void btn_clear1_Click(object sender, EventArgs e)
        {
            listBox1.Items.Clear();
        }

        private void btn_clear2_Click(object sender, EventArgs e)//清屏
        {
            txb_user.Clear();
            txb_password.Clear();
            text_rpassword.Clear();
            if (rdb_admin.Checked)
                rdb_admin.Checked = false;
            if (rdb_read.Checked) 
 
                rdb_read.Checked = false;
            if (rdb_readwrite.Checked)
                rdb_readwrite.Checked = false;
            if (rdb_write.Checked)
                rdb_write.Checked = false;
        }

        private void btn_delete_Click(object sender, EventArgs e)//删除按钮
        {
         
            listBox1.SelectedItems.Clear();
            txb_user.Clear();
            txb_password.Clear();
            text_rpassword.Clear();
            if (rdb_admin.Checked)
                rdb_admin.Checked = false;
            if (rdb_read.Checked)
                rdb_read.Checked = false;
            if (rdb_readwrite.Checked)
                rdb_readwrite.Checked = false;
            if (rdb_write.Checked)
                rdb_write.Checked = false;
        }
    }

有人帮我解决么！对你们来说很简单的 ListBox
------解决方案--------------------

n = listBox1.SelectedIndex
如果没选中任何项的话，n就会是-1，下边的listBox1.Items[n]就会异常，对n的值要先做判断处理