NullReference exception throw by FluentValidation when mocki

  using FluentAssertions;
  using FluentValidation;
  using FluentValidation.Results;
  using NSubstitute;
  using System;
  using System.Collections.Generic;

  namespace SubValidationTest
  {
  class Program
  {
    static void Main()
    {
      Test_ChildValidator();
      Test_ParentValidator_WithRealChildValidator();
      Test_ParentValidator_WithMockedChildValidator();
    }

    static void Test_ChildValidator()
    {
      var child = Substitute.For<IChild>();
      var validator = new ChildValidator();

      child.Value.Returns(null as string);
      validator.Validate(child).IsValid.Should().BeFalse();

      child.Value.Returns("");
      validator.Validate(child).IsValid.Should().BeFalse();

      child.Value.Returns("a");
      validator.Validate(child).IsValid.Should().BeTrue();
    }

    static void Test_ParentValidator_WithRealChildValidator()
    {
      var child = Substitute.For<IChild>();
      var childValidator = new ChildValidator();

      var parent = Substitute.For<IParent>();
      var validator = new ParentValidator(childValidator);

      parent.Child.Returns(null as IChild);
      validator.Validate(parent).IsValid.Should().BeTrue();

      parent.Child.Returns(child);
      validator.Validate(parent).IsValid.Should().BeFalse();

      child.Value.Returns("a");
      validator.Validate(parent).IsValid.Should().BeTrue();
    }

    static void Test_ParentValidator_WithMockedChildValidator()
    {
      var child = Substitute.For<IChild>();
      var childValidator = Substitute.For<IValidator<IChild>>();

      var parent = Substitute.For<IParent>();
      var validator = new ParentValidator(childValidator);

      parent.Child.Returns(null as IChild);
      validator.Validate(parent).IsValid.Should().BeTrue();

      parent.Child.Returns(child);

      childValidator.Validate(Arg.Any<IChild>()).Returns(new ValidationResult(new List<ValidationFailure> {  new ValidationFailure("property", "message") }));
      validator.Validate(parent).IsValid.Should().BeFalse();

      childValidator.Validate(Arg.Any<IChild>()).Returns(new ValidationResult());
      validator.Validate(parent).IsValid.Should().BeTrue();
    }
  }

    public interface IChild
    {
      string Value { get; }
    }

    public class ChildValidator : AbstractValidator<IChild>
    {
      public ChildValidator()
      {
        RuleFor(c => c.Value)
          .NotEmpty()
          .NotEmpty()
          .WithMessage("Friendly Error Message");
      }
    }

    public interface IParent
    {
      IChild Child { get; }
    }

    public class ParentValidator : AbstractValidator<IParent>
    {
      public ParentValidator(IValidator<IChild> childValidator)
      {
        When(p => p.Child != null, () => {
          RuleFor(p => p.Child)
            .SetValidator(childValidator);
        });
      }
    }
  }