C# 如果在结束窗体上选择组合框DexChanged，则我的列名中存在空引用

我正在使用combobox显示项目当我选择项目时，它在其下方的文本框中显示该项目的ID，我已在所选更改索引事件中写入代码，当我执行表单并关闭表单而不在表单中执行任何操作时，它会向我显示未设置为字符串查询中对象实例的对象引用 我的代码如下：

    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;
    using System.Data.SqlClient;

    namespace TinyErp
    { 
public partial class frmaddprod : Form
{
    public frmaddprod()
    {
        InitializeComponent();
    }


    private void productsBindingNavigatorSaveItem_Click(object sender, EventArgs e)
    {
        this.Validate();
        this.productsBindingSource.EndEdit();
        this.tableAdapterManager.UpdateAll(this.orderMachineDataSet);

    }


    SqlConnection conn = new SqlConnection(Properties.Settings.Default.OrderMachineConnectionString);
    string strgetsupp;


    private void frmaddprod_Load(object sender, EventArgs e)
    {
        // TODO: This line of code loads data into the 'orderMachineDataSet.Suppliers' table. You can move, or remove it, as needed.
        this.suppliersTableAdapter.Fill(this.orderMachineDataSet.Suppliers);
        // TODO: This line of code loads data into the 'orderMachineDataSet.Products' table. You can move, or remove it, as needed.
        this.productsTableAdapter.Fill(this.orderMachineDataSet.Products);

        //string strgetsupp = "select supplier_id from Suppliers where supplier_company='" + ((DataRowView)comboBox1.SelectedItem).Row["supplier_company"].ToString() + "'";
        //conn.Open();
        //SqlCommand cmd = new SqlCommand(strgetsupp, conn);
        //SqlDataReader read;
        //read = cmd.ExecuteReader();
        //read.Read();
        //int sid = int.Parse(read["supplier_id"].ToString());
        //read.Close();
        //conn.Close();
        //supplier_idTextBox.Text = sid.ToString();

    }

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        strgetsupp = "select supplier_id from Suppliers where supplier_company='" + ((DataRowView)comboBox1.SelectedItem).Row["supplier_company"].ToString() + "'";
        conn.Open();
        SqlCommand cmd = new SqlCommand(strgetsupp, conn);
        SqlDataReader read;
        read = cmd.ExecuteReader();
        read.Read();
        int sid = int.Parse(read["supplier_id"].ToString());
        supplier_idTextBox.Text = sid.ToString();
        read.Close();
        conn.Close();
    }

    private void productsDataGridView_CellContentClick(object sender, DataGridViewCellEventArgs e)
    {

    }

    private void frmaddprod_FormClosing(object sender, FormClosingEventArgs e)
    {
        Application.Exit();
    }
 }
 }

---

在执行任何其他操作之前，请检查comboBox1.SelectedItem是否为null

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
       if(comboBox1.SelectedItem==null)
            return;

        strgetsupp = "select supplier_id from Suppliers where supplier_company='" + ((DataRowView)comboBox1.SelectedItem).Row["supplier_company"].ToString() + "'";
        conn.Open();
        SqlCommand cmd = new SqlCommand(strgetsupp, conn);
        SqlDataReader read;
        read = cmd.ExecuteReader();
        read.Read();
        int sid = int.Parse(read["supplier_id"].ToString());
        supplier_idTextBox.Text = sid.ToString();
        read.Close();
        conn.Close();
    }

---

您知道哪一行崩溃了吗？strgetupp=“从供应商\公司='”的供应商中选择供应商\ id+（（DataRowView）comboBox1.SelectedItem）。行[“供应商\公司]。ToString（）+”；这一次它显示错误是因为出于某种原因，它试图在表单关闭时调用comboBox1\u SelectedIndexChanged。我不知道为什么。显然，当您关闭表单时，SelectedItem为null，并且不理解null.Row