title: Unable to register extension "sandbox" as extensions have already been initialized
body: <p>I have a problem installing Sonata project demo. After following <a href="https://github.com/sonata-project/sandbox" rel="nofollow">this</a>...</p>
  
  <p>I get the following problem:</p>
  
  <pre><code>Unable to register extension "sandbox" as extensions have already been initialized.
  
    in /var/www/nfq/sandbox/sonata-sandbox/vendor/twig/twig/lib/Twig/Environment.php line 627
  at Twig_Environment-&gt;addExtension(object(Twig_Extension_Sandbox))
    in /var/www/nfq/sandbox/sonata-sandbox/app/cache/dev/appDevDebugProjectContainer.php line 7111
  at appDevDebugProjectContainer-&gt;getSonata_Formatter_Twig_Env_MarkdownService()
    in /var/www/nfq/sandbox/sonata-sandbox/app/bootstrap.php.cache line 211
  </code></pre>