在线等待！System.NullReferenceException: 未将对象引用设置到对象的实例！请各位帮忙！

行 387:        int s = Convert.ToInt32(this.Teacher_Add_Class_FileUpload.PostedFile.ContentLength);我想把上传文件的大小取出来！可是到这就出现了如上的错误，找个半天，也没有找出来，所以请大家帮帮忙，解决一下！
（Teacher_Add_Class_FileUpload这个是我用的fileupload的id）
以下是我的代码：
 int s = Convert.ToInt32(this.Teacher_Add_Class_FileUpload.PostedFile.ContentLength);
        string filename = Teacher_Add_Class_FileUpload.FileName;        if (Teacher_Add_Class_FileUpload.HasFile && (s>= 17024000))
        {
            this.Teacher_Add_Class_label.Text = "所上传的文件不能超过17M！";
        }
        else
        {            string after = filename.Substring(filename.LastIndexOf(".")).ToString().ToLower();
            if (after == ".mht" || after == ".htm" || after == ".html" || after == ".ppt")
            {
                SqlDataReader sqdr = sdrd_Read("select * from coursemanager where classname='" + clas + "'and starttime='" + sttime + "'and schoolteacher='" + teacher + "'");                if (sqdr.Read())
                {
                    this.Teacher_Add_Class_label.Text = "您添加的课程已经存在,请添加正确的名称";
                    sqdr.Close();                }
                else
                {
                    DateTime upTime = DateTime.Now;
                    string timename = "";
                    timename += upTime.Year.ToString();
                    timename += upTime.Month.ToString();
                    timename += upTime.Day.ToString();
                    timename += upTime.Hour.ToString();
                    timename += upTime.Minute.ToString();
                    timename += upTime.Second.ToString();                    timename = timename.Trim();
                    timename = timename + after;                    FileUpload fu = new FileUpload();
                    string savePath = Server.MapPath("~/UpClass/");
                    savePath += timename;
                    Teacher_Add_Class_FileUpload.SaveAs(savePath);
                    string user_name = Convert.ToString(Session["user_name"]);
                    change("insert into coursemanager (spid,classname,c_teacher_user_Name,starttime,schoolteacher,teacherwords,uptime) values(" + spid + ",'" + clas + "','" + user_name + "''," + sttime + "','" + teacher + "','" + tw + "','" + timename + "')");
                    this.Teacher_Add_Class_label.Text = "&_&上传成功!^_^";
                    //Response.Write("<script language=javascript>alert('&_&上传成功!^_^');</script>");
                    this.Teacher_Add_Class_nametextbox.Value = "";
                    this.Teacher_Add_Class_sttextbox.Text = "";
                    this.Add_class_teacherTB.Text = "";
                    this.Teacher_Add_Class_teacherwordstextbox.Text = "";                }
            }
            else
            {
                this.Teacher_Add_Class_label.Text = "传的文件格式只能是（*.mht，*.htm，*.html）三种格式！";
                // Response.Write("<script language=javascript>alert('上传的文件格式只能是（*.mht，*.htm，*.html）三种格式！');</script>");
                this.Teacher_Add_Class_FileUpload.Focus();
            }