rails renders index.html before building ember app

I'm trying to build a web app with rails (backend) and ember (frontend) thanks to the ember-cli-rails gem and it works but I still have a problem.

When I launch my server with "rails s" and that I go to localhost:3000, I have an error page with "ActionView::Template::Error (:frontend has failed to build: - Building)". I think this is because rails renders the index page before waiting the end of ember build (the ember build is only launched the first time I request a page to be rendered). Because after few seconds when I refresh the page, the ember welcome page is well rendered.

Is it a normal behavior ? If not, do you have an idea to solve the problem ?

Thank you !