Fix 'createQuery() on null' in TYPO3 11?

Thanks to this answer I have a viewhelper that gets an array of categories accociated with a file reference, and this works well in TYPO3 10, PHP 7.4.

After migrating to TYPO3 11, PHP 8.1, this now throws the error:

503 Call to a member function createQuery() on null

    namespace myvendor\myext\ViewHelpers;
    
    use TYPO3\CMS\Core\Resource\FileReference;
    use TYPO3\CMS\Extbase\Persistence\Generic\QueryResult;
    use TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper;
    
    /**
    *
    * @package TYPO3
    * @subpackage myext
    * @license http://www.gnu.org/licenses/gpl.html GNU General Public License, version 2 or later
    * @author Marcus Biesioroff biesior@gmail.com>
    *
    * ViewHelper for listing file's categories
    *
    * Usage:
    * {namespace toolbox=myvendor\myext\ViewHelpers}
    * or in ext_tables.php:
    * $GLOBALS['TYPO3_CONF_VARS']['SYS']['fluid']['namespaces']['myext'] = ['myvendor\myext\ViewHelpers'];
    *
    * <cvh:fileCategories file="{file}" />
    * or
    * {cvh:fileCategories(file: file)}
    */

class FileCategoriesViewHelper extends AbstractViewHelper
    {
    
        /**
         * @var \TYPO3\CMS\Extbase\Domain\Repository\CategoryRepository
         * @TYPO3\CMS\Extbase\Annotation\Inject
         */
        protected $categoryRepository ;
    
        public function initializeArguments()
        {
            parent::initializeArguments();
            $this->registerArgument('file', 'mixed', 'File');
        }
    
    
        public function render()
        {
            /** @var FileReference $fileRef */
            $fileRef = $this->arguments['file'];
            $file = $fileRef->getOriginalFile();
            $uid = $file->getUid();
            /** @var QueryResult $res */
            $res =  $this->getCategories($uid);
            return $res->toArray();
        }
    
    
        private function getCategories($uid)
        {
            $query = $this->categoryRepository->createQuery();
            $sql = "SELECT sys_category.* FROM sys_category
                INNER JOIN sys_category_record_mm ON sys_category_record_mm.uid_local = sys_category.uid AND sys_category_record_mm.fieldname = 'categories' AND sys_category_record_mm.tablenames = 'sys_file_metadata'
                INNER JOIN sys_file_metadata ON  sys_category_record_mm.uid_foreign = sys_file_metadata.uid
                WHERE sys_file_metadata.file = '" . (int)$uid . "'
                AND sys_category.deleted = 0
                ORDER BY sys_category_record_mm.sorting_foreign ASC";
            return $query->statement($sql)->execute();
        }
    }

I tried making protected $categoryRepository = ''; but it gives the same error using string instead of null. So how can I make this work on TYPO3 11?

Try the following:

use TYPO3\CMS\Core\Resource\FileReference;
use TYPO3\CMS\Extbase\Persistence\Generic\QueryResult;
use TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper;
use TYPO3\CMS\Extbase\Domain\Repository\CategoryRepository;

/**
*
* @package TYPO3
* @subpackage myext
* @license http://www.gnu.org/licenses/gpl.html GNU General Public License, version 2 or later
* @author Marcus Biesioroff biesior@gmail.com>
*
* ViewHelper for listing file's categories
*
* Usage:
* {namespace toolbox=myvendor\myext\ViewHelpers}
* or in ext_tables.php:
* $GLOBALS['TYPO3_CONF_VARS']['SYS']['fluid']['namespaces']['myext'] = ['myvendor\myext\ViewHelpers'];
*
* <cvh:fileCategories file="{file}" />
* or
* {cvh:fileCategories(file: file)}
*/

class FileCategoriesViewHelper extends AbstractViewHelper
{
    protected $categoryRepository;

    public function __construct(
        CategoryRepository $categoryRepository
    ) {
        $this->categoryRepository = $categoryRepository;
    }

    public function initializeArguments()
    {
        parent::initializeArguments();
        $this->registerArgument('file', 'mixed', 'File');
    }


    public function render()
    {
        /** @var FileReference $fileRef */
        $fileRef = $this->arguments['file'];
        $file = $fileRef;//->getOriginalFile();
        $uid = $file->getUid();
        /** @var QueryResult $res */
        $res =  $this->getCategories($uid);
        return $res->toArray();
    }


    private function getCategories($uid)
    {
        $query = $this->categoryRepository->createQuery();
        $sql = "SELECT sys_category.* FROM sys_category
            INNER JOIN sys_category_record_mm ON sys_category_record_mm.uid_local = sys_category.uid AND sys_category_record_mm.fieldname = 'categories' AND sys_category_record_mm.tablenames = 'sys_file_metadata'
            INNER JOIN sys_file_metadata ON  sys_category_record_mm.uid_foreign = sys_file_metadata.uid
            WHERE sys_file_metadata.file = '" . (int)$uid . "'
            AND sys_category.deleted = 0
            ORDER BY sys_category_record_mm.sorting_foreign ASC";
        return $query->statement($sql)->execute();
    }
}