Not all bundles have enabled corresponding display mode(s)

Problem/Motivation

Display modes are applicable at the entity type level, not the bundle level. And not all bundles have enabled corresponding display mode(s).

$form_display = $storage->load("$entity_type_id.$bundle.$mode");

So that $form_display could be NULL here, and furthermore, it's possible to set the object of EntityDisplayInterface to NULL, and this is where the bug existed.

Proposed resolution

Check NULL cases.