Ошибка Microsoft.AspNetCore.Mvc.Razor.RazorPage.Model.get вернул null. во View

Есть Модель Product

public class Product
    {
        [Key]
        public int Id { get; set; }

        [Required]
        public string Name { get; set; }
        public string Description { get; set; }
        [Range(1, int.MaxValue)]
        public double Price { get; set; }
        public string Image { get; set; }
        [Display(Name = "Category Type")]
        public int CategoryId { get; set; }
        [ForeignKey("CategoryId")]
        public virtual Category Category { get; set; }

    }

Такой контроллер Product Controller с Index и методом Get Post для добавление и редактирования продукта

public IActionResult Index()
        {
            IEnumerable<Product> objList = _db.Product;

            foreach (var obj in objList)
            {
                obj.Category = _db.Category.FirstOrDefault(u => u.Id == obj.CategoryId);
            };

            return View(objList);
        }


        //GET - UPSERT
        public IActionResult Upsert(int? id)
        {

            //IEnumerable<SelectListItem> CategoryDropDown = _db.Category.Select(i => new SelectListItem
            //{
            //    Text = i.Name,
            //    Value = i.Id.ToString()
            //});

            ////ViewBag.CategoryDropDown = CategoryDropDown;
            //ViewData["CategoryDropDown"] = CategoryDropDown;

            //Product product = new Product();

            ProductVM productVM = new ProductVM()
            {
                Product = new Product(),
                CategorySelectList = _db.Category.Select(i => new SelectListItem
                {
                    Text = i.Name,
                    Value = i.Id.ToString()
                })
            };

            if (id == null)
            {
                //this is for create
                return View(productVM);
            }
            else
            {
                productVM.Product = _db.Product.Find(id);
                if (productVM.Product == null)
                {
                    return NotFound();
                }
                return View(productVM);
            }
        }


        //POST - UPSERT
        [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Upsert(ProductVM productVM)
        {
            if (ModelState.IsValid)
            {
                var files = HttpContext.Request.Form.Files;
                string webRootPath = _webHostEnvironment.WebRootPath;

                if (productVM.Product.Id == 0)
                {
                    //Creating
                    string upload = webRootPath + WC.ImagePath;
                    string fileName = Guid.NewGuid().ToString();
                    string extension = Path.GetExtension(files[0].FileName);

                    using (var fileStream = new FileStream(Path.Combine(upload, fileName + extension), FileMode.Create))
                    {
                        files[0].CopyTo(fileStream);
                    }

                    productVM.Product.Image = fileName + extension;

                    _db.Product.Add(productVM.Product);
                }
                else
                {
                    //updating
                }


                _db.SaveChanges();
                return RedirectToAction("Index");
            }
            return View();

        }

Когда отправляю заполненные поля в контроллер в самом начале View появляется такая ошибка: System.NullReferenceException: "Object reference not set to an instance of an object."

Microsoft.AspNetCore.Mvc.Razor.RazorPage.Model.get вернул null.

@model WebApp.Models.ViewModels.ProductVM

@{
    var title = "Create Product";
}

<form method="post" enctype="multipart/form-data">
    @if (Model.Product.Id != 0)
    {
        title = "Edit Product";
        <input asp-for="Product.Id" hidden />
    }
    <div class="border p-3">

        <div class="form-group row">
            <h2 class="text-info pl-3">@title</h2>
        </div>
        <div class="row">
            <div class="col-8">
                <div class="form-group row">
                    <div class="col-4">
                        <label asp-for="Product.Name"></label>
                    </div>
                    <div class="col-8">
                        <input asp-for="Product.Name" class="form-control" />
                        <span asp-validation-for="Product.Name" class="text-danger"></span>
                    </div>