XUnit test for the response body content

I am really new to XUnit and I will appreciate some help.

Controller

I have a really simple API method that adds a bug comment

[HttpPost("{bugId}/comment")]
public async Task<IActionResult> AddComment(Guid bugId, [FromBody] AddCommentForm form)
{
    return await _bugService.AddComment(bugId, form) switch
    {
        Option<BugViewModel>.Some(var bug) => Ok(bug),
        Option<BugViewModel>.None => NotFound(),
        _ => throw new InvalidOperationException()
    };
}

Service

The BugService method AddComment looks like this

public async Task<Option<BugViewModel>> AddComment(Guid bugId, AddCommentForm form)
{
    if (await DbBugs.SingleOrDefaultAsync(x => x.Id == bugId) is { } bug)
    {
        var bugComment = new BugComment
        {
            Text = form.Text,
            Commenter = form.Commenter,
            CommentedAt = DateTime.Now,
            Bug = bug
        };

        await _dbContext.BugComments.AddAsync(bugComment);
        await _dbContext.SaveChangesAsync();

        return new Option<BugViewModel>.Some(BugViewModel.FromData(bug));
    }
    return new Option<BugViewModel>.None();
}

ViewModels

I am adding the implementation of BugViewModel and BugCommentViewModel with their static FromData methods

public class BugViewModel
{
    public Guid Id { get; set; }
    public string Title { get; set; }
    public string Description { get; set; }          
    public List<BugCommentViewModel> Comments { get; set; }

    public static BugViewModel FromData(Bug bug)
    {
        return new BugViewModel
        {
            Id = bug.Id,
            Title = bug.Title,
            Description = bug.Description,               
            Comments = bug.Comments.Select(BugCommentViewModel.FromData).ToList()
        };
    }
}

public class BugCommentViewModel
{
    public Guid Id { get; set; }
    public string Text { get; set; }
    public string Commenter { get; set; }
    public DateTime CommentedAt { get; set; }
    
    public static BugCommentViewModel FromData(BugComment comment)
    {
        return new BugCommentViewModel
        {
            Id = comment.Id,
            Text = comment.Text,
            Commenter = comment.Commenter,
            CommentedAt = comment.CommentedAt.ToLocalTime()
        };
    }
}

Testing

The Unit Tests look like this

public class BugContollerTests
{
    private class Fixture
    {
        public IBugService BugService { get; } = Substitute.For<IBugService>();
        public BugController GetSut() => new BugController(BugService);
    }

    private readonly Fixture _fixture = new Fixture();

    [Fact]
    public async Task Create_ResponseBodyIsCreatedBug()
    {
        var bug = BugViewModel.FromData(FakeBug.I.Generate());
        _fixture.BugService.Create(Arg.Any<CreateBugForm>()).Returns(bug);

        var sut = _fixture.GetSut();
        var response = Assert.IsAssignableFrom<CreatedAtActionResult>(await sut.Create(new CreateBugForm()));

        Assert.Same(bug, response.Value);
    }

    [Fact]
    public async Task AddComment_CommentIsAddedtoViewModel()
    {
        _fixture.BugService.AddComment(Arg.Any<Guid>(), Arg.Any<AddCommentForm>()).Returns(new Option<BugViewModel>.Some(new BugViewModel()));
    
        var sut = _fixture.GetSut();
        var response = Assert.IsAssignableFrom<ObjectResult>(await sut.AddComment(Guid.Empty,new AddCommentForm()));
    
        Assert.Single(((BugViewModel) response.Value).Comments);
    }
}

So, the Test method I had as an example was Create_ResponseBodyIsCreatedBug but the question I have is regarding the second test method that I have created AddComment_CommentIsAddedtoViewModel().

What I am trying to do is: you see that the BugService.AddComment -> it adds the comment with the Bug Guid we have provided and basically we return BugViewModel with that Comment added to the Comments collection.

test: that WHEN I add a BugComment it is returned in the ViewModel response and we have added exactly one Comment to the bug.
this line

var response = Assert.IsAssignableFrom<ObjectResult>(await sut.AddComment(Guid.Empty,new AddCommentForm()));

the response.Value is a BugViewModel with empty properties and the Comment property is empty without my inserted comment.

And on this line I get an exception as

Assert.Single(((BugViewModel) response.Value).Comments);

as the Comments property is null.