Neos\Neos\Controller\Module\Management\WorkspacesController::initializeUpdateAction PHP Method

    protected function initializeUpdateAction()
    {
        $converter = new PersistentObjectConverter();
        $this->arguments->getArgument('workspace')->getPropertyMappingConfiguration()->forProperty('owner')->setTypeConverter($converter)->setTypeConverterOption(PersistentObjectConverter::class, PersistentObjectConverter::CONFIGURATION_TARGET_TYPE, User::class);
        parent::initializeAction();
    }