“Value cannot be null. Parameter name: source” while add item from connected table in View

Question

in my project i have Two table: Restaurant(RestaurantId, Name, ...., Adress) and RestaurantReviews(ReviewsId, Mark, ...., RestaurantId). In my PartialView i try show Restaurant info and calculate Average Mark, but if i try calculate Average i getting error:

ArgumentNullException: Value cannot be null. (Parameter 'source')

RestaurantModel

public class Restaurant
  {
    [Key]
    public int RestaurantId{ get; set; }

    ....

    [Required(ErrorMessage = "Restaurant name is empty")]
    public string Name{ get; set; }

    ....

    public virtual ICollection<RestaurantReviews> RestaurantReviews{ get; set; }

RestaurantReviewsModel

public class RestaurantReviews
  {
    [Key]
    public int RestaurantReviewsId { get; set; }

    ....

    [Required(ErrorMessage = "Restaurant mark is empty")]
    public int Mark{ get; set; }

    [ForeignKey("Restaurant")]
    public int RestaurantId { get; set; }

    public virtual  Restaurant Restaurant {get; set; }

DbContext

 public DbSet<Restaurant> Restaurant { get; set; }
 public DbSet<RestaurantReviews> RestaurantReviews{ get; set; }

Called PartialViews

@await Html.PartialAsync("~/Views/Restaurant/RestaurantWidget.cshtml", (IEnumerable<Firma.Data.Data.CMS.Restaurant>)ViewBag.RestaurantWidget)

RestaurantWidget.cshtml

@model IEnumerable<Firma.Data.Data.CMS.Restaurant>


@foreach (var item in Model)
{
    ...
    @item.Name
    ...

    //Without this line everything is ok
    @item.RestaurantReviews.Average(x=>x.Mark)
    //Without this line everything is ok

   ...
}

Controller

public IActionResult Index()
  {
     ViewBag.RestaurantWidget=
        (
           from restaurant in _context.Restaurant
           select restaurant 
           ).Take(5).ToList();
        return View();
  }

Additional info: Each restaurant has at least one rating. There is no restaurant that would not have any rating

Answer 1

I find answer to my question, in Controller i change

public IActionResult Index()
  {
     ViewBag.RestaurantWidget=
        (
           from restaurant in _context.Restaurant
           select restaurant 
           ).Take(5).ToList();
        return View();
  }

to code below

ViewBag.RestaurantWidget= _context.Restaurant.Include(p=>p.RestaurantReviews).Take(4).ToList();