Import: do not attempt to validate multivalue custom reference fields
The importer validates the options submitted for all custom fields that allow multiple values ("checkboxes and multiselect", according to a code comment). It does this by checking for serialize=1 in the custom field definition and running a validation routine.
However, we are now setting serialize = 1 for contact reference fields as part of work to allow multivalue contact reference fields. The validation routine is now being attempted on contact reference fields, but it is not designed for this type of field.
A simple solution (proposed) is to not attempt to run this validation routine on contact reference fields.