Persist database changes with asp.net MVC

Question

I am a beginner in the field of IT, more particularly in the C # language, I use the ASP.Net MVC framework.

I am working on an online restaurant web application, I will be the administrator of the site, so it is I who manage any addition of restaurants or menus.

For the moment I am working on my two classes Restaurant and Menu, they are linked by a relation 1:n and I use the database first approach.

So my ViewModel Restaurant-Edit has a collection of Menu while Menu-Edit has an instance of Restaurant in its code.

namespace RestaurantProjet3.Models
{
  public class MenuEditee
  {
    public int IdMenu { get; set; }

    public int fk_Resto { get; set; }
    public string NomPlat { get; set; }
    public int Prix { get; set; }
    public string Description { get; set; }
    public string Categories { get; set; }
    public byte?[] Photos { get; set; }

    public virtual Restaurant Restaurant { get; set; }
  }
}

Here, the problem that arises is the following, I worked on the page of edition of the menus of my restaurants whose here is the code

[HttpPost]
public ActionResult Edit_Menu(MenuEditee menuEdit)
{
    if(!ModelState.IsValid)
    {
        return View(menuEdit);
    }

    Menu menuBd = contexteEF.Menu.Single(m => m.IdMenu == menuEdit.IdMenu);
    menuBd = AutoMapper.Mapper.Map<MenuEditee,Menu>(menuEdit,menuBd);

    contexteEF.SaveChanges();
    return null;

When I want to replace the data contained in my comic by those contained in my ViewModel, an exception occurs telling me that

"System.InvalidOperationException: Operation failed: unable to modify the relationship because one or all of the foreign key properties do not accept null values. When a modification is made to a relationship, a null value is assigned to the associated foreign key property. If the foreign key does not support null values, a new relationship must be defined, the non-null value must be assigned to the foreign key property or the unassociated object must be deleted"

and I noticed when I put a breakpoint in my code, the restaurant instance contained in my Menu-Edit ViewModel returns a null value.

This is why when mapping its content in Menu which is in my database, one of the attributes of Menu-Edit is returned empty (the restaurant instance) which creates an exception.

What should i do to avoid errors and finally be able to persist my changes in the database?