检查会话变量(null或否)会导致Handler类中的异常

我有一个情况，就像在这篇文章中描述的那样:
我怎么能得到一个会话变量的值在c#中的静态方法?

但是，这里没有静态方法(只是从IHttpHandler继承的类)

下面是我的代码:

<%@ WebHandler Language="C#" Class="Telerik.Web.Examples.FileExplorer.FilterAndDownloadFiles.Handler" %>
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Text;
using Telerik.Web.UI;
namespace Telerik.Web.Examples.FileExplorer.FilterAndDownloadFiles
{
    [RadCompressionSettings(HttpCompression = CompressionType.None)] // Disable RadCompression for this page ;
    public class Handler : IHttpHandler
    {
        #region IHttpHandler Members
        private HttpContext _context;
        private HttpContext Context
        {
            get
            {
                return _context;
            }
            set
            {
                _context = value;
            }
        }
        public void ProcessRequest(HttpContext context)
        {
            Context = context;
            string filePath = context.Request.QueryString["path"];
            filePath = context.Server.MapPath(filePath);
            if (filePath == null)
            {
                return;
            }
            System.IO.StreamReader streamReader = new System.IO.StreamReader(filePath);
            System.IO.BinaryReader br = new System.IO.BinaryReader(streamReader.BaseStream);
            byte[] bytes = new byte[streamReader.BaseStream.Length];
            br.Read(bytes, 0, (int)streamReader.BaseStream.Length);
            if (bytes == null)
            {
                return;
            }
            streamReader.Close();
            br.Close();
            string fileName = System.IO.Path.GetFileName(filePath);
            string MimeType = GetMimeType(fileName);
            string extension = System.IO.Path.GetExtension(filePath);
            char[] extension_ar = extension.ToCharArray();
            string extension_Without_dot = string.Empty;
            for (int i = 1; i < extension_ar.Length; i++)
            {
                extension_Without_dot += extension_ar[i];
            }
            //if (extension == ".jpg")
            //{ // Handle *.jpg and
            //    WriteFile(bytes, fileName, "image/jpeg jpeg jpg jpe", context.Response);
            //}
            //else if (extension == ".gif")
            //{// Handle *.gif
            //    WriteFile(bytes, fileName, "image/gif gif", context.Response);
            //}
        if (HttpContext.Current.Session["User_ID"] != null)
        {
            WriteFile(bytes, fileName, MimeType + " " + extension_Without_dot,      context.Response);
        }
        }
        /// <summary>
        /// Sends a byte array to the client
        /// </summary>
        /// <param name="content">binary file content</param>
        /// <param name="fileName">the filename to be sent to the client</param>
        /// <param name="contentType">the file content type</param>
        private void WriteFile(byte[] content, string fileName, string contentType, HttpResponse response)
        {
            response.Buffer = true;
            response.Clear();
            response.ContentType = contentType;
            response.AddHeader("content-disposition", "attachment; filename=" + fileName);
            response.BinaryWrite(content);
            response.Flush();
            response.End();
        }
        private string GetMimeType(string fileName)
        {
            string mimeType = "application/unknown";
            string ext = System.IO.Path.GetExtension(fileName).ToLower();
            Microsoft.Win32.RegistryKey regKey = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(ext);
            if (regKey != null && regKey.GetValue("Content Type") != null)
                mimeType = regKey.GetValue("Content Type").ToString();
            return mimeType;
        }
        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
        #endregion
    }
}   

在if (HttpContext.Current.Session["User_ID"] != null)的行中，当会话变量为空时，我得到以下异常:

'/'应用程序服务器错误。

对象引用未设置为对象的实例。描述:一个执行当前web时发生未处理的异常请求。有关的详细信息，请查看堆栈跟踪错误和它在代码中的起源。

Exception Details: System。NullReferenceException:对象引用不存在设置为对象的实例

我该如何解决这个问题?

public class Handler : IHttpHandler, IRequiresSessionState