[BUGFIX] Array access warning in DataHandler

This may be triggered by invalid FlexForm values
that does not contain a 'data' key for whatever
reason.

Resolves: #100733
Releases: main, 12.4, 11.5
Change-Id: I96924d65af56fd7cf2bb2a97fd4591b3c81f934f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81586
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/core/Classes/DataHandling/DataHandler.php

@@ -3756,7 +3756,7 @@ public function copyRecord_procBasedOnFieldType($table, $uid, $field, $value, $r
             $currentValue = is_string($value) ? GeneralUtility::xml2array($value) : null;
             // Traversing the XML structure, processing files:
             if (is_array($currentValue)) {
-                $currentValue['data'] = $this->checkValue_flex_procInData($currentValue['data'], [], $dataStructureArray, [$table, $uid, $field, $realDestPid], 'copyRecord_flexFormCallBack', $workspaceOptions);
+                $currentValue['data'] = $this->checkValue_flex_procInData($currentValue['data'] ?? [], [], $dataStructureArray, [$table, $uid, $field, $realDestPid], 'copyRecord_flexFormCallBack', $workspaceOptions);
                 // Setting value as an array! -> which means the input will be processed according to the 'flex' type when the new copy is created.
                 $value = $currentValue;
             }