**System.NullReferenceException: ‘Object reference not set to an instance of an object
Microsoft.AspNetCore.Mvc.Razor.RazorPage.Model.get returned null **

Here is my code

Update.cshtml

@model WebBanHang.Models.Product
@{
    Layout = "~/Views/Shared/_Layout.cshtml";
}
<form asp-action="Update" method="post" enctype="multipart/form-data">
    <h2>Update Product</h2>
    <hr />
    <input asp-for="Id" type="hidden" />

    <div class="row">
        <div class="col-md-4">
            @if (!string.IsNullOrEmpty(Model.ImageUrl))
            {
                <img src="/@Model.ImageUrl" style="width:300px" id="previewImage" />
            }
            else
            {
                <span>No image available</span>
            }
        </div>
        <div class="col-md-8">
            <div class="form-group pb-2">
                <label>Name</label>
                <input asp-for="Name" class="form-control" />
                <span asp-validation-for="Name" class="text-danger"></span>
            </div>
            <div class="form-group pb-2">
                <label>Discription</label>
<textarea asp-for="Description" class="form-control"></textarea>
                <span asp-validation-for="Description" class="text-danger"></span>
            </div>
            <div class="form-group pb-2">
                <label>Price</label>
                <input asp-for="Price" class="form-control" />
                <span asp-validation-for="Price" class="text-danger"></span>
            </div>
            <div class="form-group pb-2">
                <label>Category</label>

                <select asp-for="CategoryId" asp-items="@ViewBag.CategoryList" class="form-control">

                    <option disabled selected>==Select category===</option>
                </select>
                <span asp-validation-for="CategoryId" class="text-danger"></span>
            </div>
            @* <img src="/@Model.ImageUrl" style="width:300px" id="previewImage" /> *@
            <div class="form-group pb-2">
                <label>Product Image</label>
                <input asp-for="ImageUrl" type="file" class="form-control" />
            </div>

            <button type="submit" class="btn btn-primary"> Update </button>
            <a asp-action="Index" class="btn btn-secondary">Back to List</a>
        </div>
    </div>
</form>
@section Scripts {
    <script>
        document.querySelectorAll('input[type="file"]').forEach(input => {
            input.addEventListener('change', function (event) {
                const file = event.target.files[0];
                if (file) {
                    const reader = new FileReader();
                    reader.onload = function (e) {
                        const img = document.getElementById("previewImage");
                        img.src = e.target.result;
                    };
                    reader.readAsDataURL(file);
                }
            });
        });
    </script>
}

ProductController.cs

using WebBanHang.Models;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;

namespace ProjectName.Controllers
{
    public class ProductController : Controller
    {
        private readonly ApplicationDbContext _db;
        private readonly  Microsoft.AspNetCore.Hosting.IHostingEnvironment _hosting;
        public ProductController(ApplicationDbContext db,  Microsoft.AspNetCore.Hosting.IHostingEnvironment hosting)
        {
            _db = db;
            _hosting = hosting;
        }
        //Hiển thị danh sách sản phẩm
        public IActionResult Index()
        {
            var productList = _db.Products.Include(x => x.Category).ToList();
            return View(productList);
        }
        //Hiển thị form thêm sản phẩm mới
        public IActionResult Add()
        {
            //truyền danh sách thể loại cho View để sinh ra điều khiển DropDownList
            ViewBag.CategoryList = _db.Categories.Select(x => new SelectListItem
            {
                Value = x.Id.ToString(),
                Text = x.Name
            });
            return View();
        }

        //Xử lý thêm sản phẩm
        [HttpPost]
        public IActionResult Add(Product product, IFormFile ImageUrl)
        {
            if (ModelState.IsValid) //kiem tra hop le
            {
                if (ImageUrl != null)
                {
                    //xu ly upload và lưu ảnh đại diện
                    product.ImageUrl = SaveImage(ImageUrl);
                }
                //thêm product vào table Product
                _db.Products.Add(product);
                _db.SaveChanges();
                TempData["success"] = "Product inserted success";
                return RedirectToAction("Index");
            }
            ViewBag.CategoryList = _db.Categories.Select(x => new SelectListItem
            {
                Value = x.Id.ToString(),
                Text = x.Name
            });
            return View();
        }
        //Hiển thị form cập nhật sản phẩm
        public IActionResult Update(int id)
        {
            var product = _db.Products.Find(id);
            if (product == null)
            {
                return NotFound();
            }
            //truyền danh sách thể loại cho View để sinh ra điều khiển DropDownList
            ViewBag.CategoryList = _db.Categories.Select(x => new SelectListItem
            {
                Value = x.Id.ToString(),
                Text = x.Name
            });
            return View(product);
        }
        //Xử lý cập nhật sản phẩm
        [HttpPost]
        public IActionResult Update(Product product, IFormFile ImageUrl)
        {
            if (ModelState.IsValid) //kiem tra hop le
            {
                var existingProduct = _db.Products.Find(product.Id);
                if (ImageUrl != null)
                {
                    //xu ly upload và lưu ảnh đại diện mới
                    product.ImageUrl = SaveImage(ImageUrl);

                    //xóa ảnh cũ (nếu có)

                    if (!string.IsNullOrEmpty(existingProduct.ImageUrl))
                    {
                        var oldFilePath = Path.Combine(_hosting.WebRootPath,
                        existingProduct.ImageUrl);
                        if (System.IO.File.Exists(oldFilePath))
                        {
                            System.IO.File.Delete(oldFilePath);
                        }
                    }
                }
                else

                {
                    product.ImageUrl = existingProduct.ImageUrl;
                }
                //cập nhật product vào table Product
                existingProduct.Name = product.Name;
                existingProduct.Description = product.Description;
                existingProduct.Price = product.Price;
                existingProduct.CategoryId = product.CategoryId;
                existingProduct.ImageUrl = product.ImageUrl;
                _db.SaveChanges();
                TempData["success"] = "Product updated success";
                return RedirectToAction("Index");
            }
            ViewBag.CategoryList = _db.Categories.Select(x => new SelectListItem
            {
                Value = x.Id.ToString(),
                Text = x.Name
            });
            return View();
        }
        private string SaveImage(IFormFile image)
        {
            //đặt lại tên file cần lưu
            var filename = Guid.NewGuid().ToString() + Path.GetExtension(image.FileName);
            //lay duong dan luu tru wwwroot tren server
            var path = Path.Combine(_hosting.WebRootPath, @"images/products");
            var saveFile = Path.Combine(path, filename);
            using (var filestream = new FileStream(saveFile, FileMode.Create))
            {
                image.CopyTo(filestream);
            }
            return @"images/products/" + filename;
        }
        //Hiển thị form xác nhận xóa sản phẩm
        public IActionResult Delete(int id)
        {
            var product = _db.Products.Find(id);
            if (product == null)
            {
                return NotFound();
            }
            return View(product);
        }
        //Xử lý xóa sản phẩm
        [HttpPost, ActionName("Delete")]
        public IActionResult DeleteConfirmed(int id)
        {
            var product = _db.Products.Find(id);
            if (product == null)
            {
                return NotFound();
            }
            // xoá hình cũ của sản phẩm
            if (!String.IsNullOrEmpty(product.ImageUrl))
            {
                var oldFilePath = Path.Combine(_hosting.WebRootPath, product.ImageUrl);
                if (System.IO.File.Exists(oldFilePath))
                {
                    System.IO.File.Delete(oldFilePath);

                }
            }
            // xoa san pham khoi CSDL
            _db.Products.Remove(product);
            _db.SaveChanges();
            TempData["success"] = "Product deleted success";
            //chuyen den action index
            return RedirectToAction("Index");
        }
    }
}

I have no idea whats going on with my code. I can delete product but cant add or update it on database.

Im also searching for a lot solution on internet but seem i cant find which is the same as my problem.

Thanks for looking into this! Hope you guys a good day!