problem with EF Core Lazy load Proxy

[Luiz Fernando Bicalho]

I created this classes

    [Table("Left")]
    public class Left
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        [Key]
        public int Id { get; set; }
        [DisallowNull]
        public virtual Child Child { get; set; } = null!;
        public virtual Left? Next { get; set; }
        public virtual IList<Right> Rights { get; set; } = new List<Right>();
    }
    [Table("Child")]
    public class Child
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        [Key]
        public int Id { get; set; }
    }

    [Table("Right")]
    public class Right
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        [Key]
        public int Id { get; set; }

        public virtual IList<Left> Lefts { get; set; } = new List<Left>();

    }

And have this problem

var left1 = new Left();
left1.EnsureValidNullability();

This throw the error

If I get from the database I get no error

            var optionsBuilder = new DbContextOptionsBuilder<ModelContext>().UseLazyLoadingProxies()
                .UseSqlServer("Server=.\\sql2019;Database=Nullable;Trusted_Connection=false;user id=sa;pwd=sa;MultipleActiveResultSets=true;Encrypt=false");

using (var context = new ModelContext(optionsBuilder.Options))
{
    var left = context.Left.Find(leftid);


    if (left != null)
    {

        var x = left.Child;
        left.GetType()?.GetProperty("Child")?.SetValue(left, null);
        x = left.Child;

        left.EnsureValidNullability();
}
}