TypeError: implode(): Argument #2 ($array) must be of type ?array, string given in implode() (line 371 of advancedform/advancedform.module).

Problem/Motivation

A massive watchdog log due to this error:
TypeError: implode(): Argument #2 ($array) must be of type ?array, string given in implode() (line 371 of advancedform/advancedform.module).