Devise confirmation instructions test is not working

I am using request_confirmation_test.rb file from devise

confirmation test from devise

I have included the helper too but the file is showing errors on each line. Looks like something is missing that is necessary for the file to work correctly.This is the error that I am getting currently

ConfirmationInstructionsTest#test_set_up_sender_from_custom_mailer_defaults:
NoMethodError: undefined method `from' for nil:NilClass
    test/integration/request_confirmation_test.rb:46:in `block in <class:ConfirmationInstructionsTest>'

this is my code for testing file

require 'test_helper'

class ConfirmationInstructionsTest < ActionMailer::TestCase

  def setup
    setup_mailer
    Devise.mailer = 'Devise::Mailer'
    Devise.mailer_sender = 'johnmax.rtc@gmail.com'
  end

  def teardown
    Devise.mailer = 'Devise::Mailer'
    Devise.mailer_sender = 'please-change-me@config-initializers-devise.com'
  end

  def user
    @user ||= create_user
  end

  def mail
    @mail ||= begin
      user
      ActionMailer::Base.deliveries.first
    end
  end

  test 'email sent after creating the user' do
    assert_not_nil mail
  end

  # test 'content type should be set to html' do
  #   assert mail.content_type.include?('text/html')
  # end

  test 'send confirmation instructions to the user email' do
    mail
    assert_equal [user.email], mail.to
  end

  test 'set up sender from configuration' do
    assert_equal ['johnmax.rtc@gmail.com'], mail.from
  end

  test 'set up sender from custom mailer defaults' do
    Devise.mailer = 'Users::Mailer'
    assert_equal ['johnmax.rtc@gmail.com'], mail.from
  end

  # test 'set up sender from custom mailer defaults with proc' do
  #   Devise.mailer = 'Users::FromProcMailer'
  #   assert_equal ['johnmax.rtc@gmail.com'], mail.from
  # end

  # test 'custom mailer renders parent mailer template' do
  #   Devise.mailer = 'Users::Mailer'
  #   assert_present mail.body.encoded
  # end

  # test 'set up reply to as copy from sender' do
  #   assert_equal ['johnmax.rtc@gmail.com'], mail.reply_to
  # end

  # test 'set up reply to as different if set in defaults' do
  #   Devise.mailer = 'Users::ReplyToMailer'
  #   assert_equal ['johnmax.rtc@gmail.com'], mail.from
  #   assert_equal ['johnmax.rtc@gmail.com'], mail.reply_to
  # end

  test 'set up subject from I18n' do
    store_translations :en, devise: { mailer: { confirmation_instructions: { subject: 'Account Confirmation' } } } do
      assert_equal 'Account Confirmation', mail.subject
    end
  end

  test 'subject namespaced by model' do
    store_translations :en, devise: { mailer: { confirmation_instructions: { user_subject: 'User Account Confirmation' } } } do
      assert_equal 'User Account Confirmation', mail.subject
    end
  end

  test 'body should have user info' do
    assert_match user.email, mail.body.encoded
  end

  test 'body should have link to confirm the account' do
    host, port = ActionMailer::Base.default_url_options.values_at :host, :port

    if mail.body.encoded =~ %r{<a href=\"http://#{host}:#{port}/users/confirmation\?confirmation_token=([^"]+)">}
      assert_equal $1, user.confirmation_token
    else
      flunk "expected confirmation url regex to match"
    end
  end

  # test 'renders a scoped if scoped_views is set to true' do
  #   swap Devise, scoped_views: true do
  #     assert_equal user.email, mail.body.decoded
  #   end
  # end

  # test 'renders a scoped if scoped_views is set in the mailer class' do
  #   begin
  #     Devise::Mailer.scoped_views = true
  #     assert_equal user.email, mail.body.decoded
  #   ensure
  #     Devise::Mailer.send :remove_instance_variable, :@scoped_views
  #   end
  # end

  test 'mailer sender accepts a proc' do
    swap Devise, mailer_sender: proc { "another@example.com" } do
      assert_equal ['another@example.com'], mail.from
    end
  end
end

This is the helper file

require 'active_support/test_case'

class ActiveSupport::TestCase
  VALID_AUTHENTICATION_TOKEN = 'AbCdEfGhIjKlMnOpQrSt'.freeze

  def setup_mailer
    ActionMailer::Base.deliveries = []
  end

  def store_translations(locale, translations, &block)
    # Calling 'available_locales' before storing the translations to ensure
    # that the I18n backend will be initialized before we store our custom
    # translations, so they will always override the translations for the
    # YML file.
    I18n.available_locales
    I18n.backend.store_translations(locale, translations)
    yield
  ensure
    I18n.reload!
  end

  def generate_unique_email
    @@email_count ||= 0
    @@email_count += 1
    "test#{@@email_count}@example.com"
  end

  def valid_attributes(attributes={})
    { first_name: "usertest",
      email: generate_unique_email,
      password: '12345678',
      password_confirmation: '12345678' }.update(attributes)
  end

  def new_user(attributes={})
    User.new(valid_attributes(attributes))
  end

  def create_user(attributes={})
    User.create!(valid_attributes(attributes))
  end

  def create_admin(attributes={})
    valid_attributes = valid_attributes(attributes)
    valid_attributes.delete(:first_name)
    Admin.create!(valid_attributes)
  end

  def create_user_without_email(attributes={})
    UserWithoutEmail.create!(valid_attributes(attributes))
  end

  def create_user_with_validations(attributes={})
    UserWithValidations.create!(valid_attributes(attributes))
  end

  # Execute the block setting the given values and restoring old values after
  # the block is executed.
  def swap(object, new_values)
    old_values = {}
    new_values.each do |key, value|
      old_values[key] = object.send key
      object.send :"#{key}=", value
    end
    clear_cached_variables(new_values)
    yield
  ensure
    clear_cached_variables(new_values)
    old_values.each do |key, value|
      object.send :"#{key}=", value
    end
  end

  def clear_cached_variables(options)
    if options.key?(:case_insensitive_keys) || options.key?(:strip_whitespace_keys)
      Devise.mappings.each do |_, mapping|
        mapping.to.instance_variable_set(:@devise_parameter_filter, nil)
      end
    end
  end
end

Any help would be highly appreciated.