Must initialise $form to avoid error "Undefined index: field_XXX in...

Must initialise $form to avoid error "Undefined index: field_XXX in Drupal\field_ui\Form\EntityDisplayFormBase->copyFormValuesToEntity()"

src/Plugin/Field/FieldFormatter/TitleFormatter.php

@@ -46,6 +46,7 @@ class TitleFormatter extends BasicStringFormatter {
    * {@inheritdoc}
    */
   public function settingsForm(array $form, FormStateInterface $form_state) {
+    $form = parent::settingsForm($form, $form_state);
     $heading_options = [
       'span' => 'span',
       'div' => 'div',