at the moment AddonsOptions::Notify is called with a null this
which crashes during adding an extension

regression from...

commit 3bdc5063
Date:   Wed Jun 22 02:30:43 2016 +0200

    tdf#89329: use shared_ptr for pImpl in addonsoptions

Change-Id: Ic28951e56bb8beca2a01ef2a1864eadcf3864e5b