selecting an archive and then removing its country will lead to a 404

[peterstadler]

This surfaced while reviewing PR #285 and was documented there first.

How to reproduce:

1. click "Sleet from list" for RISM siglum on the item paneforms/details-item.xml
2. Now select a country on the popover, then select the archive, and finally remove the country information again:

This will lead to a 404 error alert although the data is processed fine.