TypeError
Return value of TYPO3\CMS\Form\Hooks\DataStructureIdentifierHook::getLanguageService() must be an instance of TYPO3\CMS\Core\Localization\LanguageService, null returned

in /var/www/site-uol11/htdocs/typo3/sysext/form/Classes/Hooks/DataStructureIdentifierHook.php line 403
     * @return LanguageService
     */
    protected function getLanguageService(): LanguageService
    {
        return $GLOBALS['LANG'];
    }
}
at TYPO3\CMS\Form\Hooks\DataStructureIdentifierHook->getLanguageService()
in /var/www/site-uol11/htdocs/typo3/sysext/form/Classes/Hooks/DataStructureIdentifierHook.php line 54
    const L10N_PREFIX = 'LLL:EXT:form/Resources/Private/Language/Database.xlf:';

    public function __construct()
    {
        $this->getLanguageService()->includeLLFile('EXT:form/Resources/Private/Language/Database.xlf');
    }

    /**
     * The data structure depends on a current form selection (persistenceIdentifier)
at TYPO3\CMS\Form\Hooks\DataStructureIdentifierHook->__construct()
in /var/www/site-uol11/htdocs/typo3/sysext/core/Classes/Utility/GeneralUtility.php line 3220
            return self::$container->get($className);
        }

        // Create new instance and call constructor with parameters
        $instance = new $finalClassName(...$constructorArguments);
        // Register new singleton instance, but only if it is not a known PSR-11 container service
        if ($instance instanceof SingletonInterface && !(self::$container !== null && self::$container->has($className))) {
            self::$singletonInstances[$finalClassName] = $instance;
        }
at TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance()
in /var/www/site-uol11/htdocs/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php line 138
        if (!empty($GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS'][self::class]['flexParsing'])
            && is_array($GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS'][self::class]['flexParsing'])) {
            $hookClasses = $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS'][self::class]['flexParsing'];
            foreach ($hookClasses as $hookClass) {
                $hookInstance = GeneralUtility::makeInstance($hookClass);
                if (method_exists($hookClass, 'getDataStructureIdentifierPreProcess')) {
                    $dataStructureIdentifier = $hookInstance->getDataStructureIdentifierPreProcess(
                        $fieldTca,
                        $tableName,
at TYPO3\CMS\Core\Configuration\FlexForm\FlexFormTools->getDataStructureIdentifier()
in /var/www/site-uol11/htdocs/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php line 785
        // ok in certain scenarios, for instance on new record rows. Those are ok to "eat" here
        // and substitute with a dummy DS.
        $dataStructureArray = ['sheets' => ['sDEF' => []]];
        try {
            $dataStructureIdentifier = $this->getDataStructureIdentifier($GLOBALS['TCA'][$table]['columns'][$field], $table, $field, $row);
            $dataStructureArray = $this->parseDataStructureByIdentifier($dataStructureIdentifier);
        } catch (InvalidParentRowException|InvalidParentRowLoopException|InvalidParentRowRootException|InvalidPointerFieldValueException|InvalidIdentifierException $e) {
        }

at TYPO3\CMS\Core\Configuration\FlexForm\FlexFormTools->traverseFlexFormXMLData()
in /var/www/site-uol11/htdocs/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php line 913
        $this->cleanFlexFormXML = [];
        // Create and call iterator object:
        $flexObj = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Configuration\FlexForm\FlexFormTools::class);
        $flexObj->reNumberIndexesOfSectionData = true;
        $flexObj->traverseFlexFormXMLData($table, $field, $row, $this, 'cleanFlexFormXML_callBackFunction');
        return $this->flexArray2Xml($this->cleanFlexFormXML, true);
    }

    /**
at TYPO3\CMS\Core\Configuration\FlexForm\FlexFormTools->cleanFlexFormXML()
in /var/www/site-uol11/htdocs/typo3conf/ext/studipol/Classes/Upgrade/FlexFormPersonOptimizationV3UpgradeWizard.php line 150
             * explicitly want to clean up the FlexForm
             */

            // cleanup FlexForm
            $xml = $this->flexformTools->cleanFlexFormXML('tt_content', 'pi_flexform', $row);
            $xmlArray = $this->flexformTools->cleanFlexFormXML;

            // result has a different format !!!
            // $xmlArray = $this->flexformService->convertFlexFormContentToArray($row['pi_flexform']);
at Uniol\Studipol\Upgrade\FlexFormPersonOptimizationV3UpgradeWizard->executeUpdate()
in /var/www/site-uol11/htdocs/typo3/sysext/install/Classes/Service/UpgradeWizardsService.php line 400
                    $performResult = true;
                }
            } else {
                // confirmation yes or non-confirmable
                $performResult = $updateObject->executeUpdate();