Adding two fixes

src/Entity/Builder/PrivateMessageViewBuilder.php

@@ -65,7 +65,7 @@ class PrivateMessageViewBuilder extends EntityViewBuilder {
   /**
    * {@inheritdoc}
    */
-  public function view(EntityInterface $entity, $viewmode = 'full', $langcode = NULL) {
+  public function view(EntityInterface $entity, $viewMode = 'full', $langcode = NULL) {
     $message = parent::view($entity, $viewMode, $langcode);
 
     $classes = ['private-message'];

src/Plugin/Field/FieldFormatter/PrivateMessageThreadMessageFormatter.php

@@ -194,11 +194,10 @@ class PrivateMessageThreadMessageFormatter extends FormatterBase implements Cont
       '#default_value' => $this->getSetting('message_order'),
     ];
 
-    $view_modes = \Drupal::entityManager()->getViewModeOptions('private_message', TRUE);
     $element['view_mode'] = [
       '#type' => 'select',
       '#title' => $this->t('Private Message view mode'),
-      '#options' => $view_modes,
+      '#options' => $this->entityManager->getViewModeOptions('private_message', TRUE),
       '#default_value' => $this->getSetting('view_mode'),
     ];