Undefined method `show_previews' for ActionMailer::Base:Class - ruby-on-rails

Question

I have this error when I try the test of mail.
NoMethodError in Rails::MailersController#index
undefined method `show_previews' for ActionMailer::Base:Class
Extracted source (around line #49):
private
def show_previews? # :doc:
ActionMailer::Base.show_previews
end
def find_preview # :doc:
I have searched in google but I can not find anything
Rails version: 5.2.1
=> Booting Puma
=> Rails 5.2.1 application starting in development
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.12.0 (ruby 2.3.3-p222), codename: Llamas in Pajamas
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://0.0.0.0:3000
Complete log:
Started GET "/rails/mailers/register_mailer/new_user" for 127.0.0.1 at 2019-01-07 13:33:30 +0100
Processing by Rails::MailersController#preview as HTML
Parameters: {"path"=>"register_mailer/new_user"}
Completed 500 Internal Server Error in 16ms (ActiveRecord: 0.0ms)
NoMethodError (undefined method `show_previews' for ActionMailer::Base:Class):
railties (5.2.1) lib/rails/mailers_controller.rb:49:in `show_previews?'
activesupport (5.2.1) lib/active_support/callbacks.rb:435:in `block in inverted_lambda'
activesupport (5.2.1) lib/active_support/callbacks.rb:178:in `block (2 levels) in halting_and_conditional'
activesupport (5.2.1) lib/active_support/callbacks.rb:178:in `each'
activesupport (5.2.1) lib/active_support/callbacks.rb:178:in `all?'
activesupport (5.2.1) lib/active_support/callbacks.rb:178:in `block in halting_and_conditional'
activesupport (5.2.1) lib/active_support/callbacks.rb:513:in `block in invoke_before'
activesupport (5.2.1) lib/active_support/callbacks.rb:513:in `each'
activesupport (5.2.1) lib/active_support/callbacks.rb:513:in `invoke_before'
activesupport (5.2.1) lib/active_support/callbacks.rb:131:in `run_callbacks'
actionpack (5.2.1) lib/abstract_controller/callbacks.rb:41:in `process_action'
actionpack (5.2.1) lib/action_controller/metal/rescue.rb:22:in `process_action'
actionpack (5.2.1) lib/action_controller/metal/instrumentation.rb:34:in `block in process_action'
activesupport (5.2.1) lib/active_support/notifications.rb:168:in `block in instrument'
activesupport (5.2.1) lib/active_support/notifications/instrumenter.rb:23:in `instrument'
activesupport (5.2.1) lib/active_support/notifications.rb:168:in `instrument'
actionpack (5.2.1) lib/action_controller/metal/instrumentation.rb:32:in `process_action'
actionpack (5.2.1) lib/action_controller/metal/params_wrapper.rb:256:in `process_action'
activerecord (5.2.1) lib/active_record/railties/controller_runtime.rb:24:in `process_action'
actionpack (5.2.1) lib/abstract_controller/base.rb:134:in `process'
actionview (5.2.1) lib/action_view/rendering.rb:32:in `process'
actionpack (5.2.1) lib/action_controller/metal.rb:191:in `dispatch'
actionpack (5.2.1) lib/action_controller/metal.rb:252:in `dispatch'
actionpack (5.2.1) lib/action_dispatch/routing/route_set.rb:52:in `dispatch'
actionpack (5.2.1) lib/action_dispatch/routing/route_set.rb:34:in `serve'
actionpack (5.2.1) lib/action_dispatch/journey/router.rb:52:in `block in serve'
actionpack (5.2.1) lib/action_dispatch/journey/router.rb:35:in `each'
actionpack (5.2.1) lib/action_dispatch/journey/router.rb:35:in `serve'
actionpack (5.2.1) lib/action_dispatch/routing/route_set.rb:840:in `call'
rack (2.0.5) lib/rack/etag.rb:25:in `call'
rack (2.0.5) lib/rack/conditional_get.rb:25:in `call'
rack (2.0.5) lib/rack/head.rb:12:in `call'
activerecord (5.2.1) lib/active_record/migration.rb:559:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
activesupport (5.2.1) lib/active_support/callbacks.rb:98:in `run_callbacks'
actionpack (5.2.1) lib/action_dispatch/middleware/callbacks.rb:26:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/debug_exceptions.rb:61:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
railties (5.2.1) lib/rails/rack/logger.rb:38:in `call_app'
railties (5.2.1) lib/rails/rack/logger.rb:26:in `block in call'
activesupport (5.2.1) lib/active_support/tagged_logging.rb:71:in `block in tagged'
activesupport (5.2.1) lib/active_support/tagged_logging.rb:28:in `tagged'
activesupport (5.2.1) lib/active_support/tagged_logging.rb:71:in `tagged'
railties (5.2.1) lib/rails/rack/logger.rb:26:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/request_id.rb:27:in `call'
rack (2.0.5) lib/rack/runtime.rb:22:in `call'
activesupport (5.2.1) lib/active_support/cache/strategy/local_cache_middleware.rb:29:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (5.2.1) lib/action_dispatch/middleware/static.rb:127:in `call'
rack (2.0.5) lib/rack/sendfile.rb:111:in `call'
railties (5.2.1) lib/rails/engine.rb:524:in `call'
puma (3.12.0) lib/puma/configuration.rb:225:in `call'
puma (3.12.0) lib/puma/server.rb:658:in `handle_request'
puma (3.12.0) lib/puma/server.rb:472:in `process_client'
puma (3.12.0) lib/puma/server.rb:332:in `block in run'
puma (3.12.0) lib/puma/thread_pool.rb:133:in `block in spawn_thread'
I don't understand why not work because I follow all the steps of the oficial guide https://guides.rubyonrails.org/action_mailer_basics.html