Mock CollectionValidator using Moq and FluentValidation - Get NullReferenceException

I am trying to mock a child collection validator, so i can write unit tests just for the validator itself.

So i have a validator that looks like this

public class ReportValidator<T> : AbstractValidator<T>
    where T : Report
{
    public ReportValidator(IValidator<Criteria> criteriaValidator) {
        RuleFor(e => e.Name).NotEmpty();

        RuleFor(e => e.MyList).SetCollectionValidator(criteriaValidator);
    }
}

I have wrote a class in my test project to mock a validator, which looks like so

public class MockValidator<T> : Mock<IValidator<T>>
    where T : class
{
    public MockValidator() 
    {
        this.Setup(x => x.Validate(It.IsAny<ValidationContext>()))
            .Returns(new ValidationResult());

        this.Setup(x => x.ValidateAsync(It.IsAny<ValidationContext>()))
            .ReturnsAsync(new ValidationResult());
    }
}

In my unit test i create my ReportValidator, by passing in a mock like so

public class TestReportValidator
{
    private ReportValidator<Report> _validator;

    [TestInitialize]
    public void TestInit()
    {
        var mockListValidator = new MockValidator<ChildObj>();

        _validator = new ReportValidator<Report>(mockListValidator.Object);
    }

}

Now i have created my unit test to make sure that a report is valid like so

[TestMethod]
public void should_return_true_when_entity_is_valid()
{
    var results = _validator.Validate(new Report
    {
        Name = "Test",
        Module =
        {
            Id = 1,
            Name = "Company"
        },
        MyList = new Collection<ChildObj>()
    });

    Assert.AreEqual(true, results.IsValid, results.IsValid ? string.Empty : results.Errors[0].ErrorMessage);
}

But i keep getting this exception

System.NullReferenceException: Object reference not set to an instance of an object.

Not sure what i am doing wrong, anyone shed some light on this?