title: symfony flashBag message not working
body: <p>I'm building a symfony 3 project and I'm trying to show a flashbag message in twig template after submit the form  to notice the user that the form has been submitted successfully but it's not working for me ! 
  here is the <strong>controller</strong> : </p>
  
  <pre><code>&lt;?php
  namespace EvalBundle\Controller;
  
  use EvalBundle\Entity\SessionEvaluation;
  use EvalBundle\Form\SessionEvaluationType;
  use Sensio\Bundle\FrameworkExtraBundle\Configuration\Method;
  use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
  use Symfony\Bundle\FrameworkBundle\Controller\Controller;
  use Symfony\Component\HttpFoundation\Request;
  
  class EvaluationController extends Controller
  {
      /**
       * @Route("/evaluation", name="evaluation_session")
       * @Method({"GET", "POST"})
       */
      public function indexAction(Request $request)
      {
          $sessioneval = new SessionEvaluation();
          $form = $this-&gt;createForm(SessionEvaluationType::class, $sessioneval);
          $form-&gt;handleRequest($request);
  
          if ($form-&gt;isSubmitted() &amp;&amp; $form-&gt;isValid()){
  
              $em = $this-&gt;getDoctrine()-&gt;getManager();
              $em-&gt;persist($sessioneval);
              $em-&gt;flush();
              $this-&gt;get('session')-&gt;getFlashBag()-&gt;add('notice', 'la session est lancée avec succes');
              unset($entity);
  
          }
          return $this-&gt;render('@Eval/Default/Evaluation/newEvaluationSession.html.twig',array(
              'form' =&gt; $form-&gt;createView()
          ));
  
  
      }
  }
  </code></pre>
  
  <p>and here is the <strong>twig</strong> template : </p>
  
  <pre><code>{% extends ':default:superAdminBase.html.twig' %}
  
  {% block menu %}
      {% include(':default:menu.html.twig') %}
  {% endblock %}
  
  {% block header %}
  {% endblock %}
  
  {% block body %}
      &lt;div class="row"&gt;
          &lt;div class="col-lg-12 col-md-12 col-sm-12"&gt;
              &lt;h1 class="page-header"&gt;
                  Lancer une nouvelle session d'évaluation
              &lt;/h1&gt;
          &lt;/div&gt;
      &lt;/div&gt;
      {% set flashbag_notices = app.session.flashBag('notice') %}
      {% if flashbag_notices is not empty %}
          &lt;div class="row"&gt;
              &lt;div class="col-lg-6"&gt;
                  &lt;ul&gt;
                      {% for notice in  flashbag_notices %}
                          {{ dump(flashbag_notices) }}
                          &lt;li&gt;{{ notice }}&lt;/li&gt;
                      {% endfor %}
                  &lt;/ul&gt;
              &lt;/div&gt;
          &lt;/div&gt;
      {% endif %}
  
      &lt;div class="row"&gt;
          &lt;div class="col-md-12"&gt;
              &lt;div class="panel panel-default"&gt;
                  &lt;div class="panel-heading"&gt;
                      Session d'évaluation des Compétences
                  &lt;/div&gt;
                  &lt;div class="panel-body"&gt;
                      &lt;div class="row"&gt;
                          &lt;div class="container"&gt;
                              &lt;div class="col-lg-12 "&gt;
                                  {{ form_start(form) }}
                                  {{ form_widget(form) }}
                                  {{ form_end(form) }}
                              &lt;/div&gt;
                          &lt;/div&gt;
                      &lt;/div&gt;
                  &lt;/div&gt;
              &lt;/div&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  {% endblock %}
  {% block javascript %}
  
  {% endblock %}
  </code></pre>
  
  <p>I've followed the doc but I don't know it's not working on my side.</p>
  
  <p>some help please ! thanks</p>