在文本框中发送空值后，HTML帮助程序无法正常工作

Question

我在最近3个小时里一直在敲我的头，我不知道出了什么问题。

我的问题是，当我在ProjectCode中提交空白值时，为什么HTML帮助程序无效。以下是我的视图和控制器？

查看：

@using Models;
@model LoginViewModel

@{
    ViewBag.Title = "Login";
    Layout = "~/Views/_MainTheme.cshtml";
}

@using (Html.BeginForm("Search", "Home", FormMethod.Get))
{
    @Html.TextBoxFor(x => x.ProjectCode)
    <input type="submit" name="Search" value="Search" />
    <br />
        @Html.ActionLink("Create", "Details", "NewSurvey")
    <br />

    <div style="display:@(Model.SurveyList.Surveys.Count > 0 ? "block" : "none");">
        <div>@Model.SurveyListTableHeader</div>
        <table border="1">
            <thead>
                <tr>
                    <td>Project Code</td>
                    <td>Project Name</td>
            </thead>
            @foreach (var survey in Model.SurveyList.Surveys)
            {
                <tr>
                    <td>@survey.Value.ProjectCode</td>
                    <td>@survey.Value.ProjectName</td>
            }
        </table>
    </div>
}

控制器：

public ActionResult Login()
    {
        Session["CurrentPage"] = ePages.Login;
        LoginViewModel login = new LoginViewModel();
        login.SurveyList = BusinessOperations.GetExistingUserSurveys("mp12672", ref ScreenMessages._Message, ref ScreenMessages._MessageStateID);
        login.SurveyListTableHeader = "Recently Accessed Surveys";
        return View("Login", login);
    }

    public ActionResult Search(LoginViewModel login)
    {
        Session["CurrentPage"] = ePages.Login;
        login.SurveyList = BusinessOperations.GetProjectSurveys(login.ProjectCode, ref ScreenMessages._Message, ref ScreenMessages._MessageStateID);
        login.SurveyListTableHeader = "Surveys matching your search";
        return View("Login", login);
    }

    public ActionResult Resume(string SurveyKey)
    {
        Session["SurveyKey"] = SurveyKey;
        return RedirectToAction("Introduction", "OMS");
    }
}

我得到的例外是：

  

异常详细信息：System.NullReferenceException：未将对象引用设置为对象的实例。

在下面一行

@Html.ActionLink("Create", "Details", "NewSurvey")