AJAX calls fails because $form_state cache is disabled

Problem/Motivation

In v3.2.8
src/Form/OpignoActivityForm.php line 120:

// Disable cache to avoid errors with serializing files.
    $form_state->disableCache();

explicitly disables the cache. But now the ajax call to populate skills_list fails because $form_state->getValues() is empty.

Possibly related if you comment out the above code:
https://www.drupal.org/project/opigno_module/issues/3377470

Steps to reproduce

1. Edit any activity
2. Click the box that says "Assign a skill to this activity"
3. Select something from the Skill Tree
4. Now try to select something from the Skill dropdown
5. <<>> The Skill dropdown does not get populated depending on what you selected in #3

Proposed resolution

Short term resolution is to comment out the $form_state->disableCache() and possibly look at https://www.drupal.org/project/opigno_module/issues/3377470 as a remediation for the file serialization problem that it (the cacheDisable()) was supposed to address.

Remaining tasks

User interface changes

API changes

Data model changes