NullReference error when adding a new model into a model property - entity-framework

Question

I have the following entity framework code first models:
public class Member {
public int ID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string City { get; set; }
public String CardNumber { get; set; }
//Foreign Key
public virtual ICollection<Favorite> Favorites { get; set; }
[NotMapped]
public List<SelectListItem> FavoriteTypes { get; set; }
public Member() {
MembersDB db = new MembersDB();
FavoriteTypes = new List<SelectListItem>();
FavoriteTypes.AddRange(db.FavoriteTypes.ToList().Select(f => new SelectListItem { Text = f.Value, Value = f.ID.ToString() }));
}
}
public class FavoriteType {
public int ID { get; set; }
public string Value { get; set; }
}
public class Favorite {
public int ID { get; set; }
public String Value { get; set; }
//Foreign Keys
public virtual FavoriteType FavoriteType { get; set; }
public virtual Member Member { get; set; }
}
This creates a 1-M relationship for FavoriteTypes -> Favorites and 1-M relations for Member -> Favorites
Within my controller action, I retrieve most of the Member's info from Session saved at a couple pages back except for the favorites info which is gathered below. Then I gather the list of ID and input values to add to my new member as so:
[HttpPost]
public ActionResult AddFavs(List<int> ID, List<string> Value) {
MembersDB db = new MembersDB();
Member newMember = (Member)Session["member"];
if (ID != null && Value != null)
{
for (int i = 0; i < ID.Count(); i++)
{
int currentID = ID[i];
var test = new Favorite();
test.FavoriteType = db.FavoriteTypes.Where(f => f.ID == currentID).FirstOrDefault();
test.Value = Value[i];
newMember.Favorites.Add(test);
}
}
While running this code I get a NullReference error on this line newMember.Favorites.Add(test);
Not entirely sure why, any help would be appreciated.
EDIT: while troubleshooting in VS, the only null properties I can find are Favorites in newMember and Member in test

Answer 1

ICollection<Favorite> Favorites is null, so you can't add items to it. You should instantiate it in the constructor of your model:
public Member()
{
Favorites = new List<Favorite>();
// ...
}
Now it's an empty collection and you can add items to it.