ArgumentError SMTP To address may not be blank - ruby-on-rails

Question

I'm getting ArgumentError SMTP "To address" may not be blank from the Mailer that I created for the test.
My code is almost same with an example in the Rails Guide that you can find in here:
https://guides.rubyonrails.org/action_mailer_basics.html
Or Ruby on Rails tutorial:
https://3rd-edition.railstutorial.org/book/account_activation_password_reset#sec-account_activation_mailer
Server log
Error performing ActionMailer::MailDeliveryJob (Job ID: fa1eac9b-a2e0-4b4b-aca7-c785141fd7ce) from Async(mailers) in 214.6ms: ArgumentError (SMTP To address may not be blank: []):
My Code
# Ignore bad email addresses and do not raise email delivery errors.
# Set this to true and configure the email server for immediate delivery to raise delivery errors.
config.action_mailer.raise_delivery_errors = true
config.action_mailer.delivery_method = :smtp
host = 'sendish.herokuapp.com'
config.action_mailer.default_url_options = { host: host }
ActionMailer::Base.smtp_settings = {
:address => 'smtp.sendgrid.net',
:port => '587',
:authentication => :plain,
:user_name => ENV['SENDGRID_USERNAME'],
:password => ENV['SENDGRID_PASSWORD'],
:domain => 'heroku.com',
:enable_starttls_auto => true
}
# app/mailers/appliction_mailer.rb
class ApplicationMailer < ActionMailer::Base
default from: 'from#example.com'
layout 'mailer'
end
# app/mailer/user_mailer.rb
class UserMailer < ApplicationMailer
default from: 'notifications#example.com'
def welcome_email(user)
#user = user
#url = 'http://example.com/login'
attachments.inline['helloworld.gif'] = File.read(Rails.root + 'public/helloworld.gif')
mail to: user.email, subject: 'Welcome to My Awesome Site'
end
end
# app/controllers/users_controller.rb
# POST /users.json
def create
#user = User.new(user_params)
respond_to do |format|
if #user.save
# Tell the UserMailer to send a welcome email after save
UserMailer.welcome_email(#user).deliver_now
format.html { redirect_to #user, notice: 'User was successfully created.' }
format.json { render :show, status: :created, location: #user }
else
format.html { render :new }
format.json { render json: #user.errors, status: :unprocessable_entity }
end
end
end
def user_params
params.require(:user).permit(:name, :email, :login)
end
I see Mailer is passing To address in my development
UserMailer#welcome_email: processed outbound mail in 26.0ms
Date: Thu, 10 Sep 2020 15:01:52 -0700
From: notifications#example.com
To: user#example.com
Subject: Welcome to My Awesome Site
Addtional Information
The issue is happening in Heroku production