TypeError: array_diff(): Argument #1 ($array) must be of type array, null given in array_diff() (line 168

Problem/Motivation

AJAX Form Validate error on empty $_SESSION variable

Steps to reproduce

1. Add an AJAX element to the registration form. E.g. Select country etc.
2. Fill AJAX-enabled form item.
3. Encounter error 500 on AJAX request.

Proposed resolution

Add check for session variable before `array_diff`