Enable Rack::LiveReload on demand to avoid JS errors.

Gemfile

@@ -144,7 +144,7 @@ group :development, :test do
   # livereload on template changes (html, js, css)
   gem 'guard',             require: false
   gem 'guard-livereload',  require: false
-  gem 'rack-livereload'
+  gem 'rack-livereload',   require: false
   gem 'rb-fsevent',        require: false
 
   # auto symlinking

config/environments/development.rb

@@ -31,6 +31,7 @@ Rails.application.configure do
   #config.assets.debug = true
   config.assets.debug = false
 
+  if defined?(Rack::LiveReload)
     # Automatically inject JavaScript needed for LiveReload
     config.middleware.insert_after(
       ActionDispatch::Static,
@@ -40,6 +41,7 @@ Rails.application.configure do
       max_delay: 10_000, # default 60_000
       live_reload_port: 35_738
     )
+  end
 
   # Asset digests allow you to set far-future HTTP expiration dates on all assets,
   # yet still be able to expire them through the digest params.