Deleting and recreating the "Location" will break the "RISM sigla select" popover

[peterstadler]

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

How to reproduce:

1. Delete the Location and re-add it on the item pane forms/details-item.xml

2. When you now click on "Select from list" for RISM siglum, the popover will be empty and instead the drop down menus will appear within the main page: