PHP Warning: Undefined array key 0

[morange]

Hello,
since upgrading from TYPO3 v10.4 to TYPO3 v11.5 I get the following error message:

Bei dem Versuch, die Elemente für Feld "Raster-Container" (PHP Warning: Undefined array key 0 in /var/www/html/public/typo3conf/ext/gridelements/Classes/Backend/TtContent.php line 223) zu verarbeiten, ist ein Fehler aufgetreten.

My TYPO3 setting is as follows:
DDEV: 1.22.6
Apache
PHP7.4
TYPO3 11.5.34

What can I do to fix the problem?
Thx, for your help.