PHP Deprecated: strtolower(): Passing null to parameter #1 …

Getting these entries in debug.log

[03-Oct-2024 11:43:43 UTC] PHP Deprecated:  strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in .../wp-content/plugins/newsletter/includes/module-base.php on line 964

This is the code:

963:    static function sanitize_gender($gender) {
964:        $gender = trim(strtolower($gender));
965:        if (empty($gender)) {
966:            return 'n';
967:        }

Seems the problem is caused when no gender is used so $gender is null before this function call.

Suggested fix: Check for null in function before using strtolower() and trim().

Newsletter 8.5.2, WordPress 6.3.5, PHP 8.1.x