Issue #3484098 by jjchinquist, roderik: fix undefined array key.

src/Plugin/CustomElementsFieldFormatter/EntityReferenceCeFieldFormatter.php

@@ -172,7 +172,7 @@ class EntityReferenceCeFieldFormatter extends CustomElementsFieldFormatterBase {
   public function settingsSummary() {
     $target_type = $this->getFieldDefinition()->getFieldStorageDefinition()->getSetting('target_type');
     $options = $this->entityDisplayRepository->getViewModeOptions($target_type);
-    $summary[] = $this->t('View mode: @mode', ['@mode' => $options[$this->configuration['mode']]] ?? $this->configuration['mode']);
+    $summary[] = $this->t('View mode: @mode', ['@mode' => $options[$this->configuration['mode']] ?? $this->configuration['mode']]);
     if (!empty($this->configuration['flatten'])) {
       $summary[] = $this->t('Add fields directly into current element.');
       if ($this->getFieldDefinition()->getFieldStorageDefinition()->getCardinality() != 1) {