IdAsReferenceMixin.getReference() is called even in portal_property_sheets migration, so we need a backward compatibility code here, otherwise we cannot safely update 'somehow migrated' instance.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@44449 20353a03-c40f-0410-a6d1-a30d3c3de9de