remove , was a debug var

src/Plugin/Field/FieldFormatter/BgImgFieldFormatter.php

@@ -228,7 +228,6 @@ class BgImgFieldFormatter extends ResponsiveImageFormatter implements ContainerF
       if (!empty($css)) {
         $current_path =  \Drupal::request()->getRequestUri();
         if(preg_match('/node\/(\d+)\/layout/', $current_path, $matches)) {
-          $n = null;
           $elements = [
             '#theme' => 'background_style',
             '#css' => $css,