Feature description

Empty items are shown in forms with no color or warning when their relationnal widget is obsolete or broken.

A simple way could be to color it to red when the relation is not available. A warning could be displayed.

Additional context

It happens all the time when copy your layer styles to another project and don't have the same relations id or when you drop and recreate a relation.

This is strongly linked to the fact that we still can't edit / copy / paste relations in QGIS.