Restore of categories and questions happens in several phases.
First, the file is scanned for which questions and categories
it contains, to work out if these are new questions which need
to be restored, or if they already exist in the database in a
place that can be used.
That code had not been updated to cope with the Moodle 4.0
versioning changes, so it is updated here (while still keeping
the code to cope with the old backup format.)
Tim Hunt