Fatal error: Class 'TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper' not found

I had a working implementation of fs_media_gallery on what I believe was TYPO3 9.5.14. After an upgrade to 9.5.16 when opening a page with the fs_media plugin on it, I get the following error message:

Fatal error: Class 'TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper' not found in /var/www/typo3/htl-villach/public/typo3conf/ext/fs_media_gallery/Classes/ViewHelpers/Widget/AbstractWidgetViewHelper.php on line 31

Maybe this has to do with:

https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/9.0/Breaking-82414-RemoveCMSBaseViewHelperClasses.html

What can I do to fix this?

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