UserFormField gibt beim Bearbeiten NULL zurück

    Error Type:
    wcf\system\exception\ErrorException
    Error Message:
    Attempt to read property "username" on null
    File:
    */lib/system/WCF.class.php (344)
    Stack Trace:
        #0 */lib/system/form/builder/field/user/UserFormField.class.php (234):
        wcf\system\WCF::handleError(2, 'Attempt to read property "username" on null', '*/lib/system/form/builder/field/user/UserFormField.class.php', 234)
        #1 */lib/system/form/builder/field/AbstractFormField.class.php (223):
        wcf\system\form\builder\field\user\UserFormField->value(0)
        #2 */lib/system/form/builder/FormDocument.class.php (645):
        wcf\system\form\builder\field\AbstractFormField->updatedObject([ 6 items ], event\data\participant\Participant, true)
        #3 */lib/form/AbstractFormBuilderForm.class.php (246):
        wcf\system\form\builder\FormDocument->updatedObject(event\data\participant\Participant, true)
        #4 */lib/form/AbstractFormBuilderForm.class.php (142):
        wcf\form\AbstractFormBuilderForm->setFormObjectData()
        #5 */lib/page/AbstractPage.class.php (339):
        wcf\form\AbstractFormBuilderForm->readData()
        #6 */lib/page/AbstractPage.class.php (122):
        wcf\page\AbstractPage->show()
        #7 */lib/system/request/Request.class.php (89):
        wcf\page\AbstractPage->__run()
        #8 */lib/system/request/RequestHandler.class.php (119):
        wcf\system\request\Request->execute()
        #9 */event/acp/index.php (8):
        wcf\system\request\RequestHandler->handle('event', true)

<?php
namespace event\acp\form;
use event\data\group\ParticipantGroupList;
use event\data\participant\ParticipantAction;
use wcf\form\AbstractFormBuilderForm;
use wcf\system\form\builder\container\FormContainer;
use wcf\system\form\builder\field\IsDisabledFormField;
use wcf\system\form\builder\field\TextFormField;
use wcf\system\form\builder\field\user\UserFormField;

class ParticipantAddForm extends AbstractFormBuilderForm {
    /**
     * @inheritDoc
     */
    public $activeMenuItem = 'event.acp.menu.link.participant.add';
    
    /**
     * @inheritDoc
     */
    public $formAction = 'create';
    
    /**
     * @inheritDoc
     */
    public $neededPermissions = ['admin.event.canManageParticipant'];
    
    /**
     * @inheritDoc
     */
    public $objectActionClass = ParticipantAction::class;
    
    /**
     * @inheritDoc
     */
    protected function createForm() {
        parent::createForm();
        
        $dataContainer = FormContainer::create('generalSection')
            ->appendChildren([
                TextFormField::create('participantName')
                    ->label('event.acp.participant.participantName')
                    ->description('event.acp.participant.participantName.description')
                    ->required()
                    ->autoFocus()
                    ->maximumLength(255)
            ]);
        
        $availabilityContainer = FormContainer::create('availabilitySection')
            ->label('event.acp.participant.availability')
            ->appendChildren([
                UserFormField::create('userID')
                    ->label('event.acp.participant.userID')
                    ->description('event.acp.participant.userID.description'),
                IsDisabledFormField::create()
                    ->label('event.acp.participant.isDisabled')
                    ->description('event.acp.participant.isDisabled.description')
            ]);
        
        $this->form->appendChildren([
            $dataContainer,
            $availabilityContainer
        ]);
    }    
}

<?php
namespace event\acp\form;
use event\data\participant\Participant;
use event\data\participant\ParticipantCache;
use wcf\system\exception\IllegalLinkException;
use wcf\system\WCF;

class ParticipantEditForm extends ParticipantAddForm {    
    /**
     * @inheritDoc
     */
    public $formAction = 'edit';
    
    /**
     * @inheritDoc
     */
    public function readParameters() {
        parent::readParameters();
        
        if (isset($_REQUEST['id'])) {
            $this->formObject = new Participant($_REQUEST['id']);
            if (!$this->formObject->participantID) {
                throw new IllegalLinkException();
            }
        } else {
            throw new IllegalLinkException();
        }
    }
}