Issue #3354999: Fix WSOD of admin/reports/updates/settings when update.settings is removed

Closes #3354999

core/modules/update/src/UpdateSettingsForm.php

@@ -89,7 +89,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#type' => 'textarea',
       '#title' => $this->t('Email addresses to notify when updates are available'),
       '#rows' => 4,
-      '#default_value' => implode("\n", $notification_emails),
+      '#default_value' => is_array($notification_emails) ? implode("\n", $notification_emails) : '',
       '#description' => $this->t('Whenever your site checks for available updates and finds new releases, it can notify a list of users via email. Put each address on a separate line. If blank, no emails will be sent.'),
     ];