C# Session ... สอบถามเรื่อง Error Object reference not set to an instance of an object. หน่อยครับ

            if (Convert.ToString(Session["Authenation"]) == "")
            {
                Response.Redirect("frmLogin.aspx");
                Response.End();
            }

HD_User.Value = Convert.ToString(Session["Authenation"]);

protected void DeletePic()
    {
        DirectoryInfo myDirInfo = new DirectoryInfo(Server.MapPath("~/IMG/uploadProduct/" + username + "/temp/"));
        //if (!myDirInfo.Exists)
        //{
        //    myDirInfo.Create();
        //}
        FileInfo[] arrFileInfo = myDirInfo.GetFiles("*.*");
        foreach (FileInfo myFileInfo in arrFileInfo)
        {
            if (myFileInfo.Exists)
            {
                myFileInfo.Delete();
            }
        }
        DirectoryInfo directoryDeleteL1 = new DirectoryInfo(Server.MapPath("~/IMG/uploadProduct/" + username + "/temp/"));
        if (directoryDeleteL1.Exists)
        {
            directoryDeleteL1.Delete();
        }

        DirectoryInfo directoryDeleteL2 = new DirectoryInfo(Server.MapPath("~/IMG/uploadProduct/" + username));
        if (directoryDeleteL2.Exists)
        {
            directoryDeleteL2.Delete();
        }
    }