This error occured when upgrading to PHP 8 in a Drupal 9 installation:
strlen(): Passing null to parameter #1 ($string) of type string is deprecated
From:
$search = \Drupal::request()->query->get('search');
if(strlen($search) > 0) {
....
}
To:
$search = \Drupal::request()->query->get('search');
if($search) {
if(strlen($search) > 0) {
....
}
}