PHP 8.x compatibility

PHP 8.x deprecation error

A PHP 8.x deprecation error is triggered when saving an override when there is no destination set in the form.

Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in src/Form/SearchOverrideForm.php on line 112

Steps to reproduce

• Open the admin UI.
• Add new override, save
• Error is, with default PHP error reporting on, displayed before the user is redirected to the overview.

Error is in following code, $destination is NULL when no destination in the query.

    $destination = \Drupal::request()->query->get('destination');
    if (strpos($destination, 'ajax_form=1')) {
      $is_ajax = TRUE;
    }

Proposed resolution

Replace the line to get the $destination with:

    $destination = \Drupal::request()->query->get('destination', '');