not checked `referred by` checkbox causes error

If in config > study > Available subject study data the checkbox referred by is not checked add new subject throws error:
similar to #442 (closed)

changed the description

changed title from not checked the checkbox referred by cause error to not checked referred by checkbox causes error

added bug label

changed milestone to %1.1.0~alpha.0

created merge request !361 (merged) to address this issue

mentioned in merge request !361 (merged)

closed with merge request !361 (merged)

mentioned in commit 861213ad