由于找不到控件而导致的ASP.net空引用异常

我有这个例外，我不知道为什么它发生了。我搜索了这个网站和其他网站，试图找到一个解决方案，但到目前为止都没有奏效。

在我的.aspx页面中有以下两个div:

<div ID="success" style="visibility:hidden" runat="server">
// buttons and textfields
</div>
<div ID="fail" style="visibility:hidden" runat="server">
// buttons and textfields
</div>

在页面加载时，我希望一个根据某些标准成为可见的。但是当我试图在代码中瞄准它们以改变它们的可见性时，它会给我一个NullReferenceException，指向试图改变它们可见性的代码。

这是我在后面使用的代码。

fail.Style.Add("visibility", "hidden");
success.Style.Add("visibility", "Visible");

我也试过了:

fail.Attributes.Add("style", "visibility:Visible");
success.Attributes.Add("style", "visibility:Visible");

我已经在另一个。aspx页面上做了同样的操作，它没有给我这个NullReferenceException在该页上。所以我不知道发生了什么。有人能帮忙吗?

---

newBin.aspx.cs

代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Data.SqlClient;
using System.Data;
using System.Data.Common;
namespace SMTInventory
{
    public partial class newBin : Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (Page.PreviousPage != null)
            {
                HiddenField pkid_box = (HiddenField)Page.PreviousPage.FindControl("locationID");
                string pkid = pkid_box.Value;
                locationID.Value = pkid;
                success.Style.Add("visibility", "Visible");
            }
            else
            {
                if (Page.FindControl("fail") != null)
                {
                    fail.Style.Add("visibility", "hidden");
                    success.Style.Add("visibility", "Visible");
                }
                else
                {
                    fail.Style.Add("visibility", "hidden");
                    success.Style.Add("visibility", "Visible");
                }
            }
        }
        protected void btnNewLocation_Click(object sender, EventArgs e)
        {
            Server.Transfer("newLocation.aspx", true);
        }
        protected void btnNewBin_Click(object sender, EventArgs e)
        {
        }
    }
}

newBin.aspx的代码

<%@ Page Title="New Bin" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="newBin.aspx.cs" Inherits="SMTInventory.newBin" %>
<asp:Content runat="server" ID="BodyContent" ContentPlaceHolderID="MainContent">
    <hgroup class="title">
        <h1><%: Title %>.</h1>
        <h2>Add a New Bin</h2>
    </hgroup>
    <article>
        <div ID="success" style="visibility:hidden" runat="server">
            <asp:HiddenField ID="locationID" value="" runat="server" />
            How many racks are in this bin? <asp:TextBox ID="binCount" runat="server" /><br />
            <asp:button id="btnNewBin" onclick="btnNewBin_Click" runat="server" text="Add Bin" />
        </div>
        <div ID="fail" style="visibility:hidden" runat="server">
            <p>This page cannot be used without first creating a new location.<br /></p>
            <asp:Button ID="btnNewLocation" onclick="btnNewLocation_Click" runat="server" Text="Create New Location" />
        </div>
    </article>
    <aside>
        <h3>Aside Title</h3>
        <p>        
            Use this area to provide additional information.
        </p>
        <ul>
            <li><a runat="server" href="~/">Home</a></li>
            <li><a runat="server" href="~/About">About</a></li>
            <li><a runat="server" href="~/Contact">Contact</a></li>
        </ul>
    </aside>
</asp:Content>

网站的一部分。

<div id="body">
        <asp:LoginView runat="server" ViewStateMode="Disabled">
            <AnonymousTemplate>
                <asp:ContentPlaceHolder runat="server" ID="FeaturedContent" />
                <section class="content-wrapper main-content clear-fix">
                    <asp:ContentPlaceHolder runat="server" ID="MainContent" />
                </section>
                <asp:ContentPlaceHolder runat="server" ID="NewContent" />
            </AnonymousTemplate>
            <LoggedInTemplate>
            </LoggedInTemplate>
        </asp:LoginView>
    </div>

HtmlGenericControl _fail = (HtmlGenericControl)Page.FindControl("fail");
_fail.Style.Item("visibility") = "hidden";

LoginView temp = (LoginView)this.Master.FindControl("LoginView1");
ContentPlaceHolder tempp = (ContentPlaceHolder)temp.FindControl("MainContent");
HtmlGenericControl _fail = (HtmlGenericControl)tempp.FindControl("fail");

LoginView temp = (LoginView)this.Master.FindControl("LoginView1");
ContentPlaceHolder tempp = (ContentPlaceHolder)temp.FindControl("MainContent");