title: How to show FlashMessage in TYPO3 extension
body: <p>I am searching a way to show FlashMessage when user added wrong input. This piece of code is not working. thanks.</p>
  
  <pre><code>$t3type = \TYPO3\CMS\Core\Messaging\FlashMessage::NOTICE;
  $header = "Notice";
  $mes = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Messaging\FlashMessage::class,
              $message,
              $header, 
              $t3type, 
              true);
  </code></pre>