Fix missng preview_source helper error

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    lookbook (0.1.0)
+    lookbook (0.1.1)
       actioncable
       listen
       rails

app/controllers/lookbook/browser_controller.rb

@@ -1,11 +1,8 @@
-
-
 module Lookbook
   class BrowserController < ActionController::Base
     protect_from_forgery with: :exception
     prepend_view_path File.expand_path("../../views/lookbook", __dir__)
 
-    helper Lookbook::Engine.helpers
     layout "layouts/app"
 
     before_action :assign_previews

app/helpers/lookbook/application_helper.rb

@@ -16,5 +16,8 @@ def highlight(source, language)
       lexer = Rouge::Lexer.find(language)
       formatter.format(lexer.lex(source)).html_safe
     end
+
+    def preview_source
+    end
   end
 end

lib/lookbook/engine.rb

@@ -56,6 +56,11 @@ class Engine < Rails::Engine
       end
     end
 
+    config.to_prepare do
+      Lookbook::BrowserController.helper Lookbook::Engine.helpers
+      Lookbook::BrowserController.prepend_view_path Lookbook::Engine.root.join("app/views")
+    end
+
     config.after_initialize do |app|
       if app.config.lookbook.auto_refresh
         @listener = Listen.to(*app.config.lookbook.listen_paths, only: /\.(rb|html.*)$/) do |modified, added, removed|