Fallback default thumbnail in instance status API (#6177)

app/helpers/routing_helper.rb

@@ -4,6 +4,7 @@ module RoutingHelper
   extend ActiveSupport::Concern
   include Rails.application.routes.url_helpers
   include ActionView::Helpers::AssetTagHelper
+  include Webpacker::Helper
 
   included do
     def default_url_options
@@ -17,6 +18,10 @@ module RoutingHelper
     URI.join(root_url, source).to_s
   end
 
+  def full_pack_url(source, **options)
+    full_asset_url(asset_pack_path(source, options))
+  end
+
   private
 
   def use_storage?

app/serializers/rest/instance_serializer.rb

@@ -27,7 +27,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   end
 
   def thumbnail
-    full_asset_url(instance_presenter.thumbnail.file.url) if instance_presenter.thumbnail
+    instance_presenter.thumbnail ? full_asset_url(instance_presenter.thumbnail.file.url) : full_pack_url('preview.jpg')
   end
 
   def stats