cmsPageExists example

        if ($newDefaultCmsPageId === null && $salesChannelId === null) {
            $oldCmsPageId = $this->getCurrentOverallDefaultCmsPageId($systemConfigKey);

            throw CmsException::overallDefaultSystemConfigDeletion($oldCmsPageId);
        }

        if (!\is_string($newDefaultCmsPageId) && $newDefaultCmsPageId !== null) {
            throw new PageNotFoundException('invalid page');
        }

        // prevent changing the default to an invalid cms page id         if (\is_string($newDefaultCmsPageId) && !$this->cmsPageExists($newDefaultCmsPageId)) {
            throw new PageNotFoundException($newDefaultCmsPageId);
        }
    }

    private function getCurrentOverallDefaultCmsPageId(string $systemConfigKey): string
    {
        $result = $this->connection->fetchOne(
            'SELECT configuration_value FROM system_config WHERE configuration_key = :configKey AND sales_channel_id is NULL;',
            [
                'configKey' => $systemConfigKey,
            ]
        );