fix the issue

src/Plugin/Field/FieldType/ImageStyleNormalizedFieldItemList.php

@@ -21,8 +21,9 @@ protected function computeValue() {
     $config = \Drupal::config('jsonapi_image_styles.settings');
     $styles = [];
     $entity = $this->getEntity();
-    $uri = ($entity instanceof File && substr($entity->getMimeType(), 0, 5) === 'image') ? $entity->getFileUri() : FALSE;
-
+    $mimeType = $entity instanceof File ? $entity->getMimeType() : null;
+    $uri = ($mimeType && mb_substr($mimeType, 0, 5) === 'image') ? $entity->getFileUri() : FALSE;
+  
     if ($uri) {
       $defined_styles = $config->get('image_styles') ?? [];
       if (!empty(array_filter($defined_styles))) {
@@ -33,7 +34,7 @@ protected function computeValue() {
       else {
         $styles = ImageStyle::loadMultiple();
       }
-
+  
       $uris = [];
       foreach ($styles as $name => $style) {
         if ($style instanceof ImageStyle) {
@@ -42,6 +43,6 @@ protected function computeValue() {
       }
       $this->list[] = $this->createItem(0, $uris);
     }
-  }
+  }  
 
 }