How to generate a flash message in TYPO3 6.x+

TYPO3.flashMessage.php

<?php
        ....

        // creating the message
        /** @var \TYPO3\CMS\Core\Messaging\FlashMessage $message */
        $message = GeneralUtility::makeInstance(
            'TYPO3\\CMS\\Core\\Messaging\\FlashMessage',
            'Falscher Fehler',
            '',
            \TYPO3\CMS\Core\Messaging\FlashMessage::WARNING
        );

        // getting flashMessageService instance (Singleton)
        /** @var \TYPO3\CMS\Core\Messaging\FlashMessageService $flashMessageService */
        $flashMessageService = GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Messaging\\FlashMessageService');
        
        // adding message to default queue
        $flashMessageService->getMessageQueueByIdentifier()->addMessage($message);
        
        ...