TYPO3 Core

Correction: not the pi_flexform field is checked rather defined flexform data structure file.

Anyway, at least accessing 'sheets' key which may not exist should be fail-safe

if (!is_array($dataStructureArray['sheets'] ?? null)) {

Thank's for your support Garvin!

Some flexforms DS in my TYPO3 installation consist only of one sheet, so there is no <sheets> node like the one example file in your first link (DataStructureOfSingleSheet.xml). And that's the cause why mentioned PHP warning is thrown when such flexform DS files are processed by the traverse-function.

I understand that this is a huge task to backport refactored code from v13 to v12 - that's not in my mind either.
The typ3-check (!is_array) is already included in the code - I just added the part to omit PHP warning if key is not set in that array. The code afterwards wouldn't be executed with or without this change.

I see, thanks for getting back so quickly. :) So you tried a patch and that seems to work with your installation, do I understand right?

If so, would you like to contribute the patch and we could see to get it into the v12 version then, if you feel confident that it's just a small type-safety check?

Patch set 2 for branch 12.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/86366

While refactoring FlexFormTools to work with single and multi sheet flexform DS, I figured out that this job is already done in GeneralUtility:xml2Array method.
I took another round in debugging with my database and focus on CE's with flexform which trigger those PHP warnings, disovering one CE's with invalid flexform DS - missing <T3DataStructure> node at the beginning of file. By adding this node the reference indexer ran through all content without a single warning.

So this issue is solved and not a single line of code had to be touched.