Your declaration of models are wrong.Try is as below.
public class Supplier
{
public int ID { get; set; }
public virtual ICollection<SupplierItem> Items { get; set; }
}
public class Person
{
public int ID { get; set; }
public string FName { get; set; }
public string LName { get; set; }
public string Cnic { get; set; }
public string Contact { get; set; }
public DateTime DateAdded { get; set; }
public virtual Supplier Supplier { get; set; }
}
Check this article for more info :
One-to-One Foreign Key Associations