dont try to generate blob.signed_url when attachment is not saved

Author: arashnd

lib/rails_admin/config/fields/types/active_storage.rb

@@ -61,7 +61,7 @@ def resource_url(thumb = false)
 
           def value
             attachment = super
-            attachment if attachment&.attached?
+            attachment if !attachment&.new_record? && attachment&.attached?
           end
         end
       end