Use injected messenger service if possible. (#4177)

Author: Norman Kämper-Leymann

templates/module/src/Form/form.php.twig

@@ -114,7 +114,11 @@ class {{ class_name }} extends FormBase {% endblock %}
   public function submitForm(array &$form, FormStateInterface $form_state) {
     // Display result.
     foreach ($form_state->getValues() as $key => $value) {
+{% if 'messenger' in services|keys %}
+      $this->messenger->addMessage($key . ': ' . ($key === 'text_format'?$value['value']:$value));
+{% else %}
       \Drupal::messenger()->addMessage($key . ': ' . ($key === 'text_format'?$value['value']:$value));
+{% endif %}
     }
   }
 {% endblock %}