[BUGFIX] Avoid undefined array key in FlexFormTools

Resolves: #98607
Releases: main, 11.5
Change-Id: Ib57659fb5e343449de877cf5e78f55c8656bf9e9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76119
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>

typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php

@@ -766,7 +766,7 @@ public function traverseFlexFormXMLData($table, $field, $row, $callBackObj, $cal
         // Traverse languages:
         foreach ($dataStructureArray['sheets'] as $sheetKey => $sheetData) {
             // Render sheet:
-            if (is_array($sheetData['ROOT']) && is_array($sheetData['ROOT']['el'])) {
+            if (isset($sheetData['ROOT']['el']) && is_array($sheetData['ROOT']['el'])) {
                 $PA['vKeys'] = ['DEF'];
                 $PA['lKey'] = 'lDEF';
                 $PA['callBackMethod_value'] = $callBackMethod_value;