PDF template picker don't work for hidden templates

Step to reproduce.

1. Take a clean wiki without any custom settings
2. Go on administration in "PDF Export section"
2. Remove the default value for the template settings (previously you should have the default template which is listed, which is a hidden page)
3. Save and leave the page
4. Come back again to the page
5. Try again to add the default template.

Expected.

The default template should be suggested (even if it's a hidden page)

Currently.

Nothing is suggested.

Note that this issue also happen for the export tools extension which also have the template hidden. Marking the page as "not hidden" make it the picker working.