It's the first time my .Include() doesn't return the entity. I don't know what is missing.
Billings class
public Order Order { get; set; }public List<BillingItem>? BillingItems { get; set; }ApplicationDbContext:
modelBuilder.Entity<Billing>() .HasOne(e => e.Order) .WithOne(e => e.Billing) .HasForeignKey<Billing>(e => e.OrderID) .OnDelete(DeleteBehavior.Restrict);