macOS: Throw NullReferenceException when Handle is nullptr

[Carti-it]

This is a part of Felipe's PR which I slightly refactored. Felipe's original idea is captured by this snippet: https://github.com/kekyo/FlashCap/pull/171/changes#diff-828f820312b1a627c03e0e1ad658644159d006400607c23da042e4d5609d8865R59-R71. I used a guard method ValidateHandle(nameof(<FromWhichMethodCalled>));

The goal of this PR is to create a smaller more digestible chunk of #171 which is quite big for review and I believe it needs a bit more work to finish it.

cc @ffquintella
cc @kekyo

[kekyo]

Thank you both, @ffquintella and @Carti-it!

I briefly looked at Carti's PR. Is it correct to interpret these as further modifications to some of Felipe's fixes?

Since Carti's PR has its branch starting point set to develop HEAD, I'm not entirely sure how to proceed. Is it correct to interpret that applying Carti's PR after applying Felipe's PR is the better approach?

[Carti-it]

I briefly looked at Carti's PR. Is it correct to interpret these as further modifications to some of Felipe's fixes?

This PR is more like "one of Felipe's ideas extracted from #174 to a smaller PR" so one can ideally trivially say "It looks OK to me or Felipe's approach is better".

To answer simply, I think it makes sense to review this PR first and #175 second.

(If this PR is merged, Felipe or me can work on incorporating changes to #174 (I would simply create a new PR). I have some time on my hand, so I can do it. Help from @ffquintella is welcome, of course.)