diff -u b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
--- b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
+++ b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
@@ -120,8 +120,8 @@
     $element['content_types'] = array(
       '#type' => 'checkboxes',
       '#title' => t('Content Types for Internal Links'),
-      '#default_value' => $this->getSetting('content_types'),
-      '#options' => $contentTypesList
+      '#default_value' => $this->getSetting('content_types') ?: [],
+      '#options' => $contentTypesList,
     );
 
     return $element;