Exception: Warning: Undefined array key "#other_option"

Problem/Motivation

When running a custom Functional Test which uses this module.
Exception: Warning: Undefined array key "#other_option" Drupal\select_or_other\Element\ElementBase::addSelectField()() (Line: 141) is thrown

This is because $element['#other_option'] could be undefined.

Steps to reproduce

Proposed resolution

Use nullsafety check $element['#other_option'] ?? ''

Remaining tasks

User interface changes

API changes

Data model changes