ASP.NET Session返回null

Question

我正在开发asp.net web app（Web-Forms），其中我使用Session来存储用户相关数据。虽然我在尝试检索时将数据存储在会话对象中，但却给出了错误：

Object reference not set to an instance of an object.

这是我使用方法的课程：

public partial class CheckoutReview : System.Web.UI.Page, IHttpHandler, IRequiresSessionState

这里是出现错误的代码：

public partial class CheckoutReview : System.Web.UI.Page, IHttpHandler, IRequiresSessionState
    {
        protected void Page_Load(object sender, EventArgs e) 
        {
            if (!IsPostBack)
            {
                NVPAPICaller payPalCaller = new NVPAPICaller();
                string retMsg = "";
                string token = "";
                string PayerID = "";
                NVPCodec decoder = new NVPCodec();
                token = Session["token"].ToString(); <---

设置Session [“token”]：

public partial class CheckoutStart : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            NVPAPICaller payPalCaller = new NVPAPICaller();
            string retMsg = "";
            string token = "";

            if (Session["payment_amt"] != null)
            {
                string amt = Session["payment_amt"].ToString();

                bool ret = payPalCaller.ShortcutExpressCheckout(amt, ref token, ref retMsg);
                if (ret)
                {
                    Session["token"] = token;
                    Response.Redirect(retMsg);
                }
                else
                {
                    Response.Redirect("CheckoutError.aspx?" + retMsg);
                }
            }
            else
            {
                Response.Redirect("CheckoutError.aspx?ErrorCode=AmtMissing");
            }
        }
    }

如何解决此问题？