为什么我的添加按钮返回一个空引用错误

我有几个按钮在我的web应用程序，我想用它来添加/删除从一个框到另一个。使用下面的代码在过去工作，我收到一个空引用错误

错误指向-1;(int i = lbAppsI.Items.Count - 1;I>= 0;我——)在

    protected void Button1_Click(object sender, EventArgs e)
    {
        for (int i = lbAppsI.Items.Count - 1; i >= 0; i--)
        {
            if (lbAppsI.Items[i].Selected == true)
            {
                lbImpactedApps.Items.Add(new ListItem(lbAppsI.Items[i].Text, lbAppsI.Items     
 [i].Value + ","));
            }
      }
    }

未设置为对象实例的对象引用将抛出错误。

整个代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.IO;
using System.Web.UI.HtmlControls;
using System.Collections;
using System.Configuration;
namespace LandscapeServices
{
public partial class Update : System.Web.UI.Page
{
    public string query { get; set; }
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["LandscapeServicesConnectionString"].ConnectionString);
    DataSet dt = new DataSet();
    string[] IApps;
    string[] SApps;
    ListBox lbImpactedApps;
    ListBox lbSupportingApps;
    ListBox lbAppsI;
    ListBox lbAppsS;
    TextBox txtPrj;
    TextBox txtArt;

    protected void Page_Load(object sender, EventArgs e)
    {
        con.Open();
    }

    protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
    {
        GridView1.EditIndex = e.NewEditIndex;
        string ImpactedApp = ((System.Web.UI.WebControls.Label)(((System.Web.UI.WebControls.GridView)(sender)).Rows[e.NewEditIndex].FindControl("Label10"))).Text;
        IApps = ImpactedApp.Split(',');
        string SupportingApp = ((System.Web.UI.WebControls.Label)(((System.Web.UI.WebControls.GridView)(sender)).Rows[e.NewEditIndex].FindControl("Label11"))).Text;
        SApps = SupportingApp.Split(',');
    }
    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            if ((e.Row.RowState & DataControlRowState.Edit) > 0)
            {
                lbImpactedApps = (ListBox)e.Row.FindControl("ListBox1");
                lbSupportingApps = (ListBox)e.Row.FindControl("ListBox2");
                lbAppsI = (ListBox)e.Row.FindControl("ListBox3");
                lbAppsS = (ListBox)e.Row.FindControl("ListBox4");
                txtPrj = (TextBox)e.Row.FindControl("txtProjectName");
                foreach (string lst in IApps)
                {
                    lbImpactedApps.Items.Add(lst.ToString());
                    //lbImpactedApps.Items.FindByText(lst.ToString()).Selected = true;
                }
                lbImpactedApps.DataBind();
                foreach (string lst in SApps)
                {
                    lbSupportingApps.Items.Add(lst.ToString());
                    //lbSupportingApps.Items.FindByText(lst.ToString()).Selected = true;
                }
                lbSupportingApps.DataBind();
            }
        }
    }
    protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        for (int i = lbAppsI.Items.Count - 1; i >= 0; i--)
        {
            if (lbAppsI.Items[i].Selected == true)
            {
                lbImpactedApps.Items.Add(new ListItem(lbAppsI.Items[i].Text, lbAppsI.Items[i].Value + ","));
            }
      }
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
    }
    protected void Button3_Click(object sender, EventArgs e)
    {
    }
    protected void Button4_Click(object sender, EventArgs e)
    {
    }
}
}

for (int i = lbAppsI.Items.Count - 1; i >= 0; i--)