My update function is not working and get exception throw System.NullReferenceException

Question

I work on an E-commerce website using .NET as my language and LINQ as database but when I run the code my update function did not work and get exception throw System.NullReferenceException

I find my id from database

  [HttpGet]
        public ActionResult Suasach(int id)
        {
            SACH sach = data.SACHes.SingleOrDefault(n => n.Masach == id);
           
            if (sach == null)
            {
                Response.StatusCode = 404;
                return null;
            }
            ViewBag.MaCD = new SelectList(data.CHUDEs.ToList().OrderBy(n => n.TenChuDe), "MaCD", "TenChude");
            ViewBag.MaNXB = new SelectList(data.NHAXUATBANs.ToList().OrderBy(n => n.TenNXB), "MaNXB", "TenNXB");
            return View(sach);
        }

My view is this:

@model QlBanSach.Models.SACH

@{
    ViewBag.Title = "Suasach";
    Layout = "~/Views/Shared/_LayoutAdmin.cshtml";
}

<h2>ĐIỀU CHỈNH THÔNG TIN SÁCH</h2>

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()
    
<div class="form-horizontal">
    <hr />
    @Html.ValidationSummary(true)
    @Html.HiddenFor(model => model.Masach)

    

    <div class="form-group">
        @Html.LabelFor(model => model.Tensach, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.Tensach, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.Tensach, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Giaban, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.Giaban, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.Giaban, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Mota, new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            <textarea name="Mota">@Model.Mota</textarea>
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Anhbia, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">Chọn ảnh mới
        <input type="file" name="fileUpload"/>
        <img src="@Url.Content("~/images/"+Model.Anhbia)" width="120px"/>(Ảnh hiện tại)
           @ViewBag.Thongbao
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Ngaycapnhat, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            <input type="date" name="Ngaycapnhat" />
            @Html.ValidationMessageFor(model => model.Ngaycapnhat, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Soluongton, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.Soluongton, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.Soluongton, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.MaCD, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
           @Html.DropDownList("MaCD")
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.MaNXB, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
           @Html.DropDownList("MaNXB")
        </div>
    </div>

    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" value="Sửa" class="btn btn-default" />
        </div>
    </div>
</div>
}

<div>
    @Html.ActionLink("VỀ TRANG QUẢN LÝ SÁCH", "Sach")
</div>

My update function is this:

[HttpPost]
        [ValidateInput(false)]
        public ActionResult Suasach(SACH sach, HttpPostedFileBase fileupload)
        {
            ViewBag.MaCD = new SelectList(data.CHUDEs.ToList().OrderBy(n => n.TenChuDe), "MaCD", "TenChude");
            ViewBag.MaNXB = new SelectList(data.NHAXUATBANs.ToList().OrderBy(n => n.TenNXB), "MaNXB", "TenNXB");

            // TODO: Add update logic here
            if (fileupload == null)
            {
                ViewBag.Thongbao = "Vui lòng chọn ảnh bìa";
                return View();
            }
            else
            {
                if (ModelState.IsValid)
                {
                    var fileName = Path.GetFileName(fileupload.FileName);
                    var path = Path.Combine(Server.MapPath("~/images"), fileName);
                    if (System.IO.File.Exists(path))
                    {
                        ViewBag.Thongbao = "Hình ảnh đã tồn tại";
                    }
                    else
                    {
                        fileupload.SaveAs(path);
                    }
                    sach.Anhbia = fileName;
                    UpdateModel(sach);
                    data.SubmitChanges();
                }
            }
            return RedirectToAction("Sach");


        }

But when I run it my update function did not work, it got exception throw System.NullReferenceException 'Object reference not set to an instance of an object.'

System.Web.Mvc.WebViewPage<TModel>.Model.get returned null.

Is there a solution to this?