Problem filtering first level options

Here's a re-roll of #5 against 4.0.x.

I ran into the same issue today. #9 appears to resolve the issue.

The patch in #7 💬 Problem filtering first level options Needs review solved the issue that I was seeing with a 4-level taxonomy in 4.0.0 with the following settings:

Parent: (none)
Hierarchy depth: 3
Required depth: 1

I am no longer seeing the error message "The hierarchy depth cannot be 3 because the selection list has 0 levels." that I was seeing without the patch.

Patch #5 works for version 8.x-3.6.

However if you work with an older version of PHP (7.3 in my case) it's broken.

Here's a version where I remove the typed property to make it work: protected ?int $depth; => protected $depth;

Can you please re-roll this patch to the latest 4.x version?

I made a really simple but working solution. please test it.

Hmm, #15 looks like the simplest/easiest solution.

With that said, patch #12 had test coverage.

Keep in mind, the HEAD of 4.0.x isn't passing tests at this time. There was some D11 refactoring done to the tests due to deprecated PHPUnit code and so far the refactoring hasn't been straight forward and isn't completed in HEAD.