See 87627
\TYPO3\CMS\Extbase\Mvc\Controller\AbstractController::$extensionName has been removed and is no longer available in subclasses of \TYPO3\CMS\Extbase\Mvc\Controller\AbstractController, i.e. \TYPO3\CMS\Extbase\Mvc\Controller\ActionController and their derivates.
Accessing the missing property $extensionName will throw a fatal error.
All installations that read from \TYPO3\CMS\Extbase\Mvc\Controller\AbstractController::$extensionName.
The extension name is set in and available through the request object that is available in the controller. See \TYPO3\CMS\Extbase\Mvc\Controller\AbstractController::$request and \TYPO3\CMS\Extbase\Mvc\Request::getControllerExtensionName() for more information.
PHP-API, NotScanned, ext:extbase