Serialization fatal error

Problem/Motivation

The idea behind the \Drupal\taxonomy_max_depth\Form\FormAltererBase::prepareFormCallback() method is really elegant and clever but the thing is that it creates a Closure which cause fatal errors when Drupal FormBuilder tries to push the form into cache as it tries to serialize it.

Steps to reproduce

1. Install Drupal
2. Enable taxonomy max depth and set in on a vocabulary
3. Add a Media field with Media Library on the same vocabulary
4. Try to create a term and to add it a media

Expected result: the Media Library opens and you can select one
Current result: the Media Library does not open and you can see a JS error in the console: Exception: Serialization "Closure" is not allowed in serialize()

Note: you can also try with a Paragraph field and try to add a paragraph item on your term.

Proposed resolution

Change the method to return a common callable and serializable array.