grails javax.servlet.ServletException: Could not resolve view with name '/index' in servlet with name 'grailsDispatcherServlet'

i'm trying to run my app using this tutorial : http://guides.grails.org/building-a-react-app/guide/index.html

there is the the git repository https://github.com/tetar998/building-a-react-app/tree/master/complete

but i m stuck with this error Could not resolve view with name '/index' in servlet with name 'grailsDispatcherServlet'

i am currently working with - groovy 2.4.11 - java 8u152 - grails 3.3.0

and in there is my controller/demo/urlmappings

package demo

class UrlMappings {

static mappings = {
    "/$controller/$action?/$id?(.$format)?"{
        constraints {
            // apply constraints here
        }
    }

    "/"(view: '/index')
    "500"(view: '/error')
    "404"(view: '/notFound')
}
}

Any suggestion?