TYPO3 Core

Please define RendererResolver-Class in an static Member (in Example $rendererResolverClassName) in \TYPO3\CMS\Fluid\ViewHelpers\FlashMessagesViewHelper

class FlashMessagesViewHelper {

     // New Static Member for RendererResolver
    protected static $rendererResolverClassName = TYPO3\CMS\Core\Messaging\FlashMessageRendererResolver::class;

public static function renderStatic(array $arguments, \Closure $renderChildrenClosure, RenderingContextInterface $renderingContext)
{
        $as = $arguments['as'];
        $queueIdentifier = $arguments['queueIdentifier'] ?? null;
        $flashMessages = $renderingContext->getControllerContext()
            >getFlashMessageQueue($queueIdentifier)>getAllMessagesAndFlush();
        if ($flashMessages === null || count($flashMessages) === 0) {
            return '';
        }

        if ($as === null) {
            return GeneralUtility::makeInstance(static::$rendererResolverClassName)
                ->resolve()
                ->render($flashMessages);
        }

        $templateVariableContainer = $renderingContext->getVariableProvider();
        $templateVariableContainer->add($as, $flashMessages);
        $content = $renderChildrenClosure();
        $templateVariableContainer->remove($as);

return $content;
    }

.....
}

So is it easier to extend the Viewhelper and change the RendererResolver-Class to an own Class

class OwnFlashMessagesViewHelper extends \TYPO3\CMS\Fluid\ViewHelpers\FlashMessagesViewHelper {

protected static $rendererResolverClassName = OwnFlashMessageRendererResolver::class;

}