Error : Value cannot be null..

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Configuration;



namespace New
{
    public partial class DownloadFile : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.QueryString.Get("ID") != null)
            {
                Response.Clear();
                Guid ID = new Guid(Request.QueryString.Get("ID"));
                BLL.FileSave myFile = new BLL.FileSave();


                Response.ContentType = "application/x-uknown";
                Response.AppendHeader("Content_Disposition", "attachment; fileName=\"" + myFile.originalName + "\"");


                if (myFile.containsFile)
                {
                    Response.BinaryWrite(myFile.fileData);
                }
                else
                {
//the error is here
                    Response.WriteFile(Path.Combine(Bll.AppConfiguration.UploadsFolder, myFile.fileUrl));
               }

            }
        }
    }
}