ارور Object reference not set to an instance of an object.'

سلام و عرض خسته نباشید در قسمت 65 بخش ساخت صفحه Services/Create زمانی که متد onPost رو فرا میخونه مقادیر پراپرتی CarServiceVM رو حذف میکنه و ارور null میده. کدها رو از فایل استاد کپی کردم بازم درست نشد فقط مقادیری که به صورت input در view وجود دارند مقدار دارند بقیه زمانی که کلید submit زده میشه حذف میشن

    public class CreateModel : PageModel
    {
        private ApplicationDbContext _db;

        public CreateModel(ApplicationDbContext db)
        {
            _db = db;
        }

        [BindProperty]
        public CarServiceViewModel CarServiceVM { get; set; }
        public async Task<IActionResult> OnGet(int carId)
        {
            CarServiceVM = new CarServiceViewModel()
            {
                Car = await _db.Cars.Include(c => c.ApplicationUser)
                    .FirstOrDefaultAsync(c => c.Id == carId),
                ServiceHeader = new ServiceHeader()
            };

            List<string> lstServiceTypeInShoppingCart = _db.ServiceShoppingCarts
                .Include(c => c.ServiceType)
                .Where(c => c.CarId == carId)
                .Select(c => c.ServiceType.Name)
                .ToList();

            IQueryable<ServiceType> lstServices = from s in _db.ServiceTypes
                                                  where !(lstServiceTypeInShoppingCart.Contains(s.Name))
                                                  select s;

            CarServiceVM.ServiceTypes = lstServices.ToList();
            CarServiceVM.ServiceShoppingCarts = _db.ServiceShoppingCarts.Include(c => c.ServiceType)
                .Where(c => c.CarId == carId)
                .ToList();
            CarServiceVM.ServiceHeader.TotalPrice = 0;

            foreach (var item in CarServiceVM.ServiceShoppingCarts)
            {
                CarServiceVM.ServiceHeader.TotalPrice += item.ServiceType.Price;
            }

            return Page();
        }


        public async Task<IActionResult> OnPost()
        {
            if (ModelState.IsValid)
            {
                CarServiceVM.ServiceHeader.Date = DateTime.Now;
                CarServiceVM.ServiceShoppingCarts = _db.ServiceShoppingCarts.Include(c => c.ServiceType)
                    .Where(c => c.CarId == CarServiceVM.Car.Id).ToList();
                foreach (var shop in CarServiceVM.ServiceShoppingCarts)
                {
                    CarServiceVM.ServiceHeader.TotalPrice += shop.ServiceType.Price;
                }

                CarServiceVM.ServiceHeader.CarId = CarServiceVM.Car.Id;
                _db.ServiceHeaders.Add(CarServiceVM.ServiceHeader);
                await _db.SaveChangesAsync();

                foreach (var shop in CarServiceVM.ServiceShoppingCarts)
                {
                    ServiceDetails details = new ServiceDetails()
                    {
                        ServiceHeaderId = CarServiceVM.ServiceHeader.Id,
                        ServiceName = shop.ServiceType.Name,
                        ServicePrice = shop.ServiceType.Price,
                        ServiceTypeId = shop.ServiceTypeId
                    };
                    _db.ServiceDetails.Add(details);
                }
                _db.ServiceShoppingCarts.RemoveRange(CarServiceVM.ServiceShoppingCarts);
                await _db.SaveChangesAsync();
                return RedirectToPage("/Cars/Index", new { userId = CarServiceVM.Car.UserId });
            }

            return Page();
        }