Spent again a whole day trying to understand what happens here, with no luck. For some reasons, OperatorProperties children of unregistered operator classes remain accessible, with their `bl_rna` member, although that one is 100% invalid (freed memory, crashes with ASAN builds). Funny thing is, I cannot reproduce that situation when disabling the add-on from the py console of a Blender-with-UI. Note: issue revealed by X3D add-on, which is still enabled in factory settings, while not being officially supported any more, this has to be fixed in a separate commit.