Without any demo-code hard to say where the exception is being triggered but some data binding from the Add-form is not working,  in CarsController should be:

        [HttpPost]
        public HttpResponse Add(CarInputModel inputModel)
        {
            if (!this.IsUserSignedIn())
            {
                return this.Redirect("/Users/Login");
            }

            ... All necessary validations

            var carId = this.carsService.Create(inputModel.Model, inputModel.Year, inputModel.Image, inputModel.PlateNumber);

            return this.Redirect($"/Issues/CarIssues?carId={carId}");
        }

In CarsService:

        public string Create(string name, int year, string image, string plateNumber)
        {
            var car = new Car
            {
                Model = model,
                Year = year,
                Image = image,
                PlateNumber = plateNumber,
            };

            this.dbContext.Cars.Add(car);

            this.dbContext.SaveChanges();

            var carId = car.Id;

            return carId;
        }

Hope this works,