REST namespace: fatal error

[jeherve]

This is a follow-up from #329

I recently ran into the following fatal error when installing ActivityPub 1.0 on the primary site of a multisite network:

PHP Fatal error: Uncaught Error: Call to a member function using_index_permalinks() on null in wp-includes/rest-api.php:459
Stack trace:
#0 wp-content/plugins/activitypub/includes/functions.php(208): get_rest_url()
#1 wp-content/plugins/activitypub/includes/model/class-application-user.php(31): Activitypub\get_rest_url_by_path()
#2 wp-content/plugins/activitypub/includes/class-signature.php(140): Activitypub\Model\Application_User->get_url()
#3 wp-content/plugins/activitypub/includes/class-http.php(72): Activitypub\Signature::generate_signature()
#4 wp-content/plugins/activitypub/includes/functions.php(85): Activitypub\Http::get()
#5 wp-content/plugins/activitypub/includes/collection/class-followers.php(166): Activitypub\get_remote_metadata_by_actor()
#6 wp-content/plugins/activitypub/includes/class-migration.php(86): Activitypub\Collection\Followers::add_follower()
#7 wp-content/plugins/activitypub/includes/class-migration.php(68): Activitypub\Migration::migrate_from_0_17()
#8 wp-content/plugins/activitypub/includes/class-activity-dispatcher.php(27): Activitypub\Migration::maybe_migrate()
#9 wp-content/plugins/activitypub/activitypub.php(39): Activitypub\Activity_Dispatcher::init()
#10 wp-includes/class-wp-hook.php(308): Activitypub\init()