Railsでdeviseを使ったgithub認証を実装していますが、request.envが空になります

Question

Railsでdeviseを使ったgithub認証を実装していますが、request.envが空になります。
1.Githubにアクセス
2.Githubにログイン情報を入力
3.開発中のサービスに戻って来た時に以下のエラーが出ます。

この場合、どういう状況が考えられるかご教示頂けますと幸いです。

NoMethodError in Users::SessionsController#new
undefined method `provider' for nil:NilClass

• 開発環境

Rails 5.1.3
ruby 2.4.0p0
devise 4.3.0

• config/route.rb

 - [省略] -
  devise_for :users, controllers: {
      omniauth_callbacks: "users/omniauth_callbacks"
  }

  devise_scope :user do
    get "sign_in", :to => "users/sessions#new", :as => :new_session
    get "sign_out", :to => "users/sessions#destroy", as: :destroy_session
  end
 - [省略] -

• controller/users/sessions_controller.rb

class Users::SessionsController < Devise::SessionsController
    def new
        user = User.find_for_oauth(request.env['omniauth.auth'])
        session[:user_id] = user.id
        redirect_to root_path
    end

    def destroy
        reset_session
        redirect_to root_path
    end
end

• controller/users/omniauth_callback_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController

  def github
    @user = User.find_for_oauth(request.env["omniauth.auth"])

    if @user.persisted?
      sign_in_and_redirect @user, :event => :authentication #this will throw if @user is not activated
      set_flash_message(:notice, :success, :kind => "Github") if is_navigational_format?
    else
      session["devise.github_data"] = request.env["omniauth.auth"]
      redirect_to new_user_registration_url
    end
  end
end

• models/user

class User < ApplicationRecord
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :trackable, :database_authenticatable, :omniauthable, :omniauth_providers => [:github]
  
  def self.find_for_oauth(auth)
    user = User.where(provider: auth.provider, uid: auth.uid).first

    unless user
      user = User.create( name:     auth.extra.raw_info.name,
                          provider: auth.provider,
                          uid:      auth.uid,
                          email:    auth.info.email,
                          token:    auth.credentials.token,
                          password: Devise.friendly_token[0,20] )
    end

    return user
  end

  soft_deletable :column => :delete_at
end