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.
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