TypeError: trim(): Argument #1 ($string) must be of type string, array given in trim()

Problem/Motivation

TypeError: trim(): Argument #1 ($string) must be of type string, array given in trim() (line 362 of /options_element/options_element.inc).

Steps to reproduce

Use a function on a list field and go to the field settings
page:
/admin/structure/types/manage/NODE-TYPE/fields/field_FIELD_NAME/field-settings

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes