OnlineMedia Processing forces FAL drivers to extend AbstractDriver
FAL drivers are not obligated to extend the AbstractDriver but only to implement the DriverInterface. Having custom drivers that are not based on AbstractDriver (because they are f.e. non hierarchical) currently causes an Exception because \TYPO3\CMS\Core\Resource\OnlineMedia\Processing\PreviewProcessing expects an instance of AbstractDriver as method argument and not DriverInterface.
Patch/fix is already on Gerrit. Ticket only for reference.