Divers null error on BatEventSettingsForm

Problem/Motivation

In the route admin/bat/config/bat_event, when nothing is configured yet: 3 nullable warning

Warning: Trying to access array offset on null in Drupal\bat_event\Form\BatEventSettingsForm->buildForm() (line 48 of modules/contrib/bat/modules/bat_event/src/Form/BatEventSettingsForm.php).

Warning: Trying to access array offset on null in Drupal\bat_event\Form\BatEventSettingsForm->buildForm() (line 55 of modules/contrib/bat/modules/bat_event/src/Form/BatEventSettingsForm.php).

Warning: Trying to access array offset on null in Drupal\bat_event\Form\BatEventSettingsForm->buildForm() (line 65 of modules/contrib/bat/modules/bat_event/src/Form/BatEventSettingsForm.php).

Proposed resolution

Add isset test.