[BUGFIX] Avoid 'undefined array key "sheetTitle"' warning

Properly guard array key access for "sheetTitle" in
`\TYPO3\CMS\Backend\Form\Container\FlexFormTabsContainer->render()`.

Resolves: #98201
Releases: main, 11.5
Change-Id: Iafa5fd4623d53501e655739d7aabc543d26ed216
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75546


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/backend/Classes/Form/Container/FlexFormTabsContainer.php

@@ -83,7 +83,7 @@ class FlexFormTabsContainer extends AbstractContainer
 
             if ($childReturn['html'] !== '') {
                 $tabElements[] = [
-                    'label' => !empty(trim($sheetDataStructure['ROOT']['sheetTitle'])) ? $languageService->sL(trim($sheetDataStructure['ROOT']['sheetTitle'])) : $sheetName,
+                    'label' => !empty(trim($sheetDataStructure['ROOT']['sheetTitle'] ?? '')) ? $languageService->sL(trim($sheetDataStructure['ROOT']['sheetTitle'])) : $sheetName,
                     'content' => $childReturn['html'],
                     'description' => trim($sheetDataStructure['ROOT']['sheetDescription'] ?? '') ? $languageService->sL(trim($sheetDataStructure['ROOT']['sheetDescription'])) : '',
                     'linkTitle' => trim($sheetDataStructure['ROOT']['sheetShortDescr'] ?? '') ? $languageService->sL(trim($sheetDataStructure['ROOT']['sheetShortDescr'])) : '',