rails5(apiモード) × devise_token_auth × omniauth(FB)の実装について

Question

叶えたいこと

rails5(apiモード) ×　devise_token_auth × omniauth(Facebook)の処理で、無事にjsonを返却させたい

やったこと

Rails5のAPIモードでdevise_token_authとomniauthを使ったログインを試す
の記事を参考に、現在SNS認証を実装しています。

おおよその実装は上記のサイトと同じにして(users/omniauth_callbacks_controller:7のcreate_token_infoは実際のdevise_token_authのgemの実装になかったので削除)、変更点として、FacebookのSNS連携を実現させたいと思ったので、それ用の設定をomniauth.rbとomniauth.ymlに追加した程度です。

しかし、実際に動かしてみると、認証urlを叩き、DBの保存まではいくのですが、
render json: @resource, status: :ok
の処理のところで下記のエラーが発生しています。

原因がわからず詰まっておりまして、誰かアドバイスを頂けますと幸いです。
※json出力にはjbuilderではなく評判の良さそうなfast_json_apiを使用しています。
ちなみに、他の箇所で
render json: {test: 'test'}, status: :ok
と設定すると正しく返ってきたので、fast_json_apiの使い方が問題ではないような感じています。

エラー内容

NoMethodError (undefined method `downcase' for nil:NilClass):
  
rack (2.0.7) lib/rack/utils.rb:469:in `[]='
rack (2.0.7) lib/rack/utils.rb:443:in `block in initialize'
rack (2.0.7) lib/rack/utils.rb:443:in `each'
rack (2.0.7) lib/rack/utils.rb:443:in `initialize'
rack (2.0.7) lib/rack/utils.rb:437:in `new'
rack (2.0.7) lib/rack/utils.rb:437:in `new'
rack (2.0.7) lib/rack/conditional_get.rb:26:in `call'
rack (2.0.7) lib/rack/head.rb:12:in `call'
activerecord (5.2.3) lib/active_record/migration.rb:559:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
activesupport (5.2.3) lib/active_support/callbacks.rb:98:in `run_callbacks'
actionpack (5.2.3) lib/action_dispatch/middleware/callbacks.rb:26:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/debug_exceptions.rb:61:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
railties (5.2.3) lib/rails/rack/logger.rb:38:in `call_app'
railties (5.2.3) lib/rails/rack/logger.rb:26:in `block in call'
activesupport (5.2.3) lib/active_support/tagged_logging.rb:71:in `block in tagged'
activesupport (5.2.3) lib/active_support/tagged_logging.rb:28:in `tagged'
activesupport (5.2.3) lib/active_support/tagged_logging.rb:71:in `tagged'
railties (5.2.3) lib/rails/rack/logger.rb:26:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/request_id.rb:27:in `call'
rack (2.0.7) lib/rack/runtime.rb:22:in `call'
activesupport (5.2.3) lib/active_support/cache/strategy/local_cache_middleware.rb:29:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (5.2.3) lib/action_dispatch/middleware/static.rb:127:in `call'
rack (2.0.7) lib/rack/sendfile.rb:111:in `call'
rack-cors (1.1.0) lib/rack/cors.rb:100:in `call'
railties (5.2.3) lib/rails/engine.rb:524:in `call'
puma (3.12.1) lib/puma/configuration.rb:227:in `call'
puma (3.12.1) lib/puma/server.rb:660:in `handle_request'
puma (3.12.1) lib/puma/server.rb:474:in `process_client'
puma (3.12.1) lib/puma/server.rb:334:in `block in run'
puma (3.12.1) lib/puma/thread_pool.rb:135:in `block in spawn_thread'

環境

ruby:2.6.5
apiモードで作成
gemfileの中身

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '2.6.5'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.2.3'
# Use mysql as the database for Active Record
gem 'mysql2', '>= 0.4.4', '< 0.6.0'
# Use Puma as the app server
gem 'puma', '~> 3.11'

gem 'devise'
gem 'omniauth'
gem 'omniauth-facebook'
gem 'omniauth-line'

# クロスドメイン対策
# Use Rack CORS for handling Cross-Origin Resource Sharing (CORS), making cross-origin AJAX possible
gem 'rack-cors'

gem 'devise-i18n'
gem 'rails-i18n'

gem 'devise_token_auth', github: 'lynndylanhurley/devise_token_auth'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
# gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use ActiveStorage variant
# gem 'mini_magick', '~> 4.8'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '>= 1.1.0', require: false
# Use Rack CORS for handling Cross-Origin Resource Sharing (CORS), making cross-origin AJAX possible
# gem 'rack-cors'
gem 'fast_jsonapi'

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
  gem 'rspec-rails'
  gem 'factory_bot_rails'
  gem 'dotenv-rails'
end

group :development do
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
  gem 'bullet'
  gem 'pry-byebug'
  gem 'rubocop', require: false
  gem 'rubocop-performance', require: false
  gem 'rubocop-rails', require: false
end


# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]