TYPO3 Core

PHP Warning: Undefined array key "updateReferenceIndex" in /var/www/html/public/typo3/sysext/extbase/Classes/Persistence/Generic/Backend.php line 651

The $frameworkConfiguration looks like this:

array (
  'persistence' => array (
    'storagePid' => 0,
  ),
  'controllerConfiguration' => array (),
)

Solution:

if (($frameworkConfiguration['persistence']['updateReferenceIndex'] ?? null) === '1') {
  // ...
}

TYPO3 v11.5.17
PHP v8.0