Ajax error because of PHP 7.4, Trying to access array offset on value of type null

Re-roll of #9 for 3.x.

The patch from #11 works well here! Thank you!

I'm getting this error with PHP 8.1, D 9.5, and field_states_ui 3.0.0.

#11 seems to resolve it. Thank you!

Updating the title as this isn't just PHP 7.4.
Taking the leap here and moving to RTBC, it is being reported as valid, patch is small and looks good to me. Thank you @godotislate.

RTBC + 1 -> Working on Drupal 10.3.1

Tested patch #11 with v3.0.0 on Drupal core 10.2.11 and PHP 8.1.29 ... resolves warning messages.

Patch #11 unfortunately did not worked for me anymore. I am on Drupal Version 10.3.10, PHP 8.1.30. Drupal log was flooded with messages as

Warning: Undefined array key "simple_sitemap" in Drupal\field_ui\Form\EntityDisplayFormBase->copyFormValuesToEntity() (Zeile 643 in /var/www/html/web/core/modules/field_ui/src/Form/EntityDisplayFormBase.php).

and

Warning: Trying to access array offset on value of type null in Drupal\field_ui\Form\EntityDisplayFormBase->copyFormValuesToEntity() (Zeile 643 in /var/www/html/web/core/modules/field_ui/src/Form/EntityDisplayFormBase.php).

Even worse, on my local installation, the fields on content types' form display page (admin/structure/types/manage//form-display) could not be edited anymore, showing more and more of these messages on top of the page, leading to:

*Drupal\Core\Database\DatabaseExceptionWrapper*: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT 1 AS "expression" FROM "sessions" "sessions" WHERE "sid" = :db_condition_placeholder_0; Array ( [:db_condition_placeholder_0] => bSH7ZZvbRVb_7-exNS2ItOFW-U3CvuFPPFamw54fVuM ) in *Drupal\Core\Session\SessionHandler->Drupal\Core\Session\{closure}()* (line *95* of */app/web/core/lib/Drupal/Core/Session/SessionHandler.php*)

This post directed me to the right solution: Stackoverflow – Message: Trying to access array offset on value of type null
The function copyFormValuesToEntity() in core/modules/field_ui/src/Form/EntityDisplayFormBase.php did not check for value NULL and existence of array value in two places. Previous versions of PHP may have been less strict on such mistakes and silently swallowed the error / notice.
Here is a patch → that works for in our drupal installation. Hope it helps.