FileReference: only first file loaded

[rowild]

I try to add audiofiles to a post using FileReference. Eventually only the first of a max of 4 is "recignized", displaying uid and pid in the resulting json, even though uid should not be used.
This is what I do:

// Add to ext_tables.sql
CREATE TABLE tx_jsonexample_domain_model_post (
	[...],
	audiofiles int(11) unsigned NOT NULL DEFAULT '0',
);

// Add to tx_jsonexample_domain_model_post.php TCA
    'types' => [
        '1' => [  'showitem' => 'hidden, title, post_text, tags, audiofiles'  ],
    ],
    'columns' => [
        'audiofiles' => [
            'exclude' => true,
            'label' => 'Audio files',
            'config' => \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::getFileFieldTCAConfig(
                'audiofiles',
                [
                    'appearance' => [
                        'createNewRelationLinkTitle' => 'Add',
                        [...]
                    ],
                    'foreign_types' => [
                        '0' => [
                            'showitem' => '
                            --palette--;LLL:EXT:lang/locallang_tca.xlf:sys_file_reference.imageoverlayPalette;imageoverlayPalette,
                            --palette--;;filePalette'
                        ],
                        \TYPO3\CMS\Core\Resource\File::FILETYPE_TEXT => [
                           [...]
                        ],
                        [...]
                    ],
                    'foreign_match_fields' => [
                        'fieldname' => 'audiofiles',
                        'tablenames' => 'tx_jsonexample_domain_model_post',
                        'table_local' => 'sys_file',
                    ],
                    'maxitems' => 4
                ]
            ),
        ]
    ]


// Add to Domain/Model/Post

    /**
     * @var \TYPO3\CMS\Extbase\Domain\Model\FileReference
     */
    protected $audiofiles;

    public function __construct(string $title, string $postText)
    {
        [...];
        $this->audiofiles = new \TYPO3\CMS\Extbase\Domain\Model\FileReference();
    }

    /**
     * @return \TYPO3\CMS\Extbase\Domain\Model\FileReference $audiofiles
     */
    public function getAudiofiles(): \TYPO3\CMS\Extbase\Domain\Model\FileReference
    {
        return $this->audiofiles;
    }
    
// ...and eventually to Json View:

         'posts' => [
            '_descendAll' => [
                '_descend' => [
                    'tags' => [ ... ],
                    'audiofiles' => [
                        '_descendAll' => [
                            '_only' => ['uid']
                        ]
                    ]
                ]
            ]
        ],

But when calling the RestAPI URL (via Postman), TYPO3 returns this error:

(1/1) TypeError
Return value of DanielGoerz\JsonExample\Domain\Model\Post::getAudiofiles() must be an instance of TYPO3\CMS\Extbase\Domain\Model\FileReference, null returned

in /var/www/html/packages/json_example/Classes/Domain/Model/Post.php line 45

(in line 45 is the 'getAudiofiles' method)

What do I have to do to be able to get potentially all 4 audio files from the post? Ideally displaying a link to the audio file?