viewMode in Drupal\core_event_dispatcher\Event\Entity\EntityBuildDefaultsAlterEvent could be null in Drupal 9.4 somehow

Is anyone able to trace what causing this issue?

I don't think it makes sense to have no view mode, core's entity view builder itself has full default value.

public function view(EntityInterface $entity, $view_mode = 'full', $langcode = NULL);
public function viewMultiple(array $entities = [], $view_mode = 'full', $langcode = NULL);

doesn't it will give a problem later if the view mode is NULL?

Try this patch. works for me on 9.5.

Patch made compatible with hook_event_dispatcher 4.0.1

Patch made compatible with hook_event_dispatcher 4.0.2

Patch made compatible with hook_event_dispatcher 4.2.0

Postponing as this possibly due to custom code or other contrib modules. The view mode isn't meant to be null, core itself have 'full' for default value.