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.