I’m working on a project (ASP.NET MVC5) for school but today I ran into this error even the same code yesterday is exactly like this 100% but still ran and today when i open it again it run to error. I’m already trying to fix this for a long hours but nothing works. I don’t know why it null anyway, i have tried to fix a lot but still get null when i put the edit ( SuaCT in the CT View ), and also the Details ( ChiTietCT ) and Delete ( XoaCT ) is get null errors too, its seem when i click on these buttons, it can not get the data from the list to edit, details and delete.

My view SuaCT.cshtml

@model STMA.Models.CONGTHUC

@{
    ViewBag.Title = "SuaCT";
}

<h2>SuaCT</h2>

<link href="~/Content/SuaCT.css" rel="stylesheet" type="text/css" />

<center><h2 style="font-family: Georgia, serif; margin-top: 30px;">Sửa Công Thức</h2></center>

<div class="container-fluid">
    <form class="col-lg-12" method="post" action="SuaCT">
        <center>
            <div class="col-md-5">
                <div>
                    <select name="MaMA" aria-label="Chọn Món Ăn">
                        <option>@Model.MaMA</option> <!--Errors here-->
                    </select>
                </div>
                <div>
                    <select name="MaNL" aria-label="Chọn Nguyên Liệu">
                        <option>@Model.MaNL</option>
                    </select>
                </div>
                <div>
                    <input type="number" min="0" name="TrongLuong" value="" />
                </div>
                <div>
                    <select name="DVT" aria-label="Chọn DVT" class="select">
                        <optgroup label="Trọng Lượng">
                            <option value="gram">gram</option>
                            <option value="mg">mg</option>
                            <option value="kg">kg</option>
                        </optgroup>
                        <optgroup label="Thể Tích">
                            <option value="ml">ml</option>
                            <option value="lít">l</option>
                        </optgroup>
                    </select>
                </div>
                <div class="col-md-offset-2 col-md-10">
                    <input type="submit" value="✎" class="btn btn-default" />
                    <a href="@Url.Action("CT")" class="btn">➜</a>
                </div>
            </div>
        </center>
    </form>
</div>

@section Scripts {
    @Scripts.Render("~/bundles/jqueryval")
}

My CongThuc Controller

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using STMA.Models;

namespace STMA.Controllers
{
    public class CongThucController : Controller
    {
        // GET: CongThuc
        // GET: Last

        TIEULUANEntities data = new TIEULUANEntities();
        public ActionResult CT()
        {
            return View(data.CONGTHUCs.ToList());
        }

        public ActionResult ThemCT()
        {
            ViewBag.dropNL = new SelectList(data.NGUYENLIEUx, "MaNL", "TenNL");
            ViewBag.dropMA = new SelectList(data.MONANs, "MaMA", "TenMA");
            return View();
        }

        [HttpPost]
        public ActionResult ThemCT(CONGTHUC ct)
        {
            ViewBag.dropNL = new SelectList(data.NGUYENLIEUx, "MaNL", "TenNL");
            ViewBag.dropMA = new SelectList(data.MONANs, "MaMA", "TenMA");
            data.CONGTHUCs.Add(ct);
            data.SaveChanges();
            return RedirectToAction("CT");
        }

        public ActionResult SuaCT(string mact1, string mact2)
        {
            CONGTHUC ct = data.CONGTHUCs.Find(new object [] { mact1, mact2 });
            return View(ct);
        }

        [HttpPost]
        public ActionResult SuaCT(string mact1, string mact2, CONGTHUC ct)
        {
            CONGTHUC ctt = data.CONGTHUCs.Find( new object[] { mact1 , mact2 });
            if (ctt == null)
            {
                return HttpNotFound();
            }
            // Gán giá trị cho các thuộc tính của ctt từ ct
            ctt.TrongLuong = ct.TrongLuong;
            ctt.DVT = ct.DVT;

            // Lưu thay đổi vào cơ sở dữ liệu
            data.SaveChanges();

            return RedirectToAction("CT");
        }
        public ActionResult ChiTietCT(string mact1, string mact2)
        {
            ViewBag.dropNL = new SelectList(data.NGUYENLIEUx, "MaNL", "TenNL");
            ViewBag.dropMA = new SelectList(data.MONANs, "MaMA", "TenMA");
            CONGTHUC ct = data.CONGTHUCs.Find(new object[] { mact1, mact2 });
            return View(ct);
        }
        public ActionResult XoaCT(string mact1, string mact2)
        {
            CONGTHUC ct = data.CONGTHUCs.Find( new object[] { mact1, mact2});
            data.CONGTHUCs.Remove(ct);
            data.SaveChanges();
            return RedirectToAction("CT");
        }
    }
}

My List View

@model IEnumerable<STMA.Models.CONGTHUC>

@{
    ViewBag.Title = "CT";
}

<center><h2 class="my-5" style="font-family: Georgia, serif;">Danh Sách Công Thức</h2></center>

<p>
    <a href="@Url.Action("ThemCT","CongThuc")" name="ThemCT" class="btn btn-outline-warning text-dark mx-2"><b>Thêm</b></a>
</p>

<div class="container-fluid" style="font-size: 17px;">
    <div class="row">
        <div class="col-lg-12">
            <table class="table table-borderless" style="overflow: auto">
                <thead>
                    <tr class="col-lg-12 table-dark">
                        <th>
                            <label>Tên Món</label>
                        </th>
                        <th>
                            <label>Tên Nguyên Liệu</label>
                        </th>
                        <th>
                            <label>Trọng Lượng</label>
                        </th>
                        <th>
                            <label>Đơn Vị Tính</label>
                        </th>
                        <th class="col-1"></th>
                    </tr>
                </thead>

                <tbody>
                    @foreach (var item in Model)
                    {
                    <tr class="col-lg-12">
                        <td>
                            @Html.DisplayFor(Model => item.MONAN.TenMA)
                        </td>
                        <td>
                            @Html.DisplayFor(Model => item.NGUYENLIEU.TenNL)
                        </td>
                        <td>
                            @item.TrongLuong
                        </td>
                        <td>
                            @item.DVT
                        </td>
                        <td class="btn-group" role="group">
                            <a class="btn btn-success" href="@Url.Action("SuaCT", "CongThuc", new { mact1 = item.MaMA , mact2 = item.MaNL })">✎</a>
                            <a class="btn btn-primary" href="@Url.Action("ChiTietCT", "CongThuc", new {mact1 = item.MaMA , mact2 = item.MaNL })">☰</a>
                            <a class="btn btn-danger" href="@Url.Action("XoaCT", "CongThuc", new { mact1 = item.MaMA , mact2 = item.MaNL })">⨉</a>
                        </td>
                    </tr>
                    }
                </tbody>
            </table>
        </div>
    </div>
</div>