Problem/Motivation
If a site builder deletes the media library view mode and then uses the UI to add another media type, a fatal error is thrown upon saving the form.
[29-Aug-2019 07:55:32 America/New_York] Error: Call to a member function getConfigDependencyName() on null in /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Entity/EntityDisplayBase.php on line 314 #0 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php(319): Drupal\Core\Entity\EntityDisplayBase->calculateDependencies()
#1 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Entity/EntityDisplayBase.php(266): Drupal\Core\Config\Entity\ConfigEntityBase->preSave(Object(Drupal\Core\Config\Entity\ConfigEntityStorage))
#2 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php(491): Drupal\Core\Entity\EntityDisplayBase->preSave(Object(Drupal\Core\Config\Entity\ConfigEntityStorage))
#3 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php(446): Drupal\Core\Entity\EntityStorageBase->doPreSave(Object(Drupal\Core\Entity\Entity\EntityViewDisplay))
#4 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Config/Entity/ConfigEntityStorage.php(263): Drupal\Core\Entity\EntityStorageBase->save(Object(Drupal\Core\Entity\Entity\EntityViewDisplay))
#5 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Entity/EntityBase.php(395): Drupal\Core\Config\Entity\ConfigEntityStorage->save(Object(Drupal\Core\Entity\Entity\EntityViewDisplay))
#6 /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php(613): Drupal\Core\Entity\EntityBase->save()
#7 /Users/oknate/dev/d8test/web/core/modules/media_library/media_library.module(345): Drupal\Core\Config\Entity\ConfigEntityBase->save()
#8 /Users/oknate/dev/d8test/web/core/modules/media_library/media_library.module(200): _media_library_configure_view_display(Object(Drupal\media\Entity\MediaType))
#9 [internal function]: _media_library_media_type_form_submit(Array, Object(Drupal\Core\Form\FormState))
Proposed resolution
Either prevent the deletion of the media library view mode while the module is enabled, or gracefully exit _media_library_configure_view_display if the view mode isn't available.