TYPO3 Core

Your Controller overrides the injectConfigurationManager() method of ActionController. The original takes care of setting $this->arguments, which triggers the error you see

You must either call the parent method before overriding, or take care of all the things the parent performs, setting this variable.

So this is not a core bug but an implementation problem. Maybe you can actually use initializeIndexAction() to perform fetching settings.

Going to close this issue as WON'T FIX as it is related to a extension not being
TYPO3 v12 compatible. Extension should be overworked to a v12 compatible state.

To accomplish that, please
read the TYPO3 Core Changelogs carefully and in detail, regarding TYPO3 v12 but also
prior core versions as the extenion contains code deprecated already in v11 and removed
in v12.

Additionally, you may look into the TYPO3 Extension Scanner to detect deprecated stuff,
and also use rector to update your extension properly - or at least for as much as the
tool can help.

For example, ObjectManager ( $this->objectManager ) has been removed as breaking change
in TYPO3 v12, with proper long time deprecation notince already in TYPO3 v11.