Include EntityInterface in the generateBackgroundCss() function signature.

src/Plugin/Field/FieldFormatter/BgImgFieldFormatter.php

@@ -3,6 +3,7 @@
 namespace Drupal\bg_img_field\Plugin\Field\FieldFormatter;
 
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Field\FieldDefinitionInterface;
@@ -341,7 +342,7 @@ class BgImgFieldFormatter extends ResponsiveImageFormatter implements ContainerF
   /**
    * CSS Generator Helper Function.
    *
-   * @param object $image
+   * @param Drupal\Core\Entity\EntityInterface $file
    *   URI of the field image.
    * @param string $responsive_image_style
    *   Desired picture mapping to generate CSS.
@@ -353,7 +354,7 @@ class BgImgFieldFormatter extends ResponsiveImageFormatter implements ContainerF
    * @return string
    *   Generated background image CSS.
    */
-  protected function generateBackgroundCss($file, $responsive_image_style, $selector, array $options) {
+  protected function generateBackgroundCss(EntityInterface $file, $responsive_image_style, $selector, array $options) {
     $css = "";
 
     if (!$file) {