PHP 8.1+ Deprecation: strtolower() receiving null in text-contains.php

Seeing persistent PHP deprecation warnings in debug log:

PHP Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /wp-content/themes/voxel/app/post-types/field-conditions/text-contains.php on line 21

This occurs frequently when creating/updating posts with Voxel CPTs.

Environment:

Voxel version: 1.7.4
PHP version: 8.2
WordPress version: 6.9