Check if image alias uri is valid before using it

bundle/Resources/views/nglayouts/themes/admin/app/item/nglayouts_app_preview.html.twig

@@ -1,7 +1,7 @@
 {% if content.hasField('image') and not content.fields.image.empty %}
     {% set image_alias = ng_image_alias(content.fields.image, 'nglayouts_app_preview') %}
 
-    {% if image_alias %}
+    {% if image_alias and image_alias.uri is not null %}
         <div class="image">
             <img src="{{ asset(image_alias.uri) }}" />
         </div>