Fatal error: "enableFields on non-object" in extension manager
Extbase extensions might fail with:
Fatal error: Call to a member function enableFields() on a non-object in .../typo3_src/typo3/sysext/core/Classes/Resource/StorageRepository.php on line 211
The problem was "uncovered" with the #45498 (the new EM fatals with this error since this patch), but could happen in other situations too.
Reason is that Extbase in certain situations will try to create a dummy simulated "TSFE" object to be able to use cObject stdWrap's in the backend context.
Now this is not a "complete TSFE" and doesn't for example include a proper sys_page property.
So to make sure we are in a proper FE context, we shouldn't rely on "is_object($TSFE)" but instead check if TYPO3_MODE==FE like it is done throughout other places in the core.