Ruby,Railsのバージョンアップに伴うサーバ起動時のエラー

Ruby on Railsのアプリで以下の作業を行ったところ、開発環境（AWS Cloud9）内でサーバが立ち上がらずエラーになってしまいます。
エラーメッセージを調べても、糸口がいまいちつかめず解決に至らずにいます。

もし、問題ありそうな可能性という程度でも何かアドバイス頂ければ、この上なくありがたいです。

何卒よろしくお願いいたします

やったこと

以下へそれぞれバージョンアップデートを実施

　・ruby '2.3.4'　→　ruby '2.5.9'
・rails '4.2.5'　→　rails ‘4.2.8'

開発環境

AWS Cloud9

事象

開発環境内でサーバが立ち上らず、以下のエラーメッセージになる

エラーメッセージ

Traceback (most recent call last):

        24: from bin/rails:9:in `<main>'

        23: from bin/rails:9:in `require'

        22: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands.rb:17:in `<top (required)>'

        21: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:39:in `run_command!'

        20: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:75:in `server'

        19: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:75:in `tap'

        18: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:80:in `block in server'

        17: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/server.rb:78:in `start'

        16: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/server.rb:139:in `log_to_stdout'

        15: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/server.rb:337:in `wrapped_app'

        14: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/railties-4.2.8/lib/rails/commands/server.rb:61:in `app'

        13: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/server.rb:209:in `app'

        12: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/server.rb:300:in `build_app_and_options_from_config'

        11: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/builder.rb:40:in `parse_file'

        10: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/builder.rb:49:in `new_from_string'

         9: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/builder.rb:49:in `eval'

         8: from /home/ubuntu/environment/アプリ名/config.ru:in `<main>'

         7: from /home/ubuntu/environment/アプリ名/config.ru:in `new'

         6: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/builder.rb:55:in `initialize'

         5: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/rack-1.6.13/lib/rack/builder.rb:55:in `instance_eval'

         4: from /home/ubuntu/environment/アプリ名/config.ru:6:in `block in <main>'

         3: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:274:in `require'

         2: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:240:in `load_dependency'

         1: from /home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:274:in `block in require'


/home/ubuntu/アプリ名/vendor/bundle/ruby/2.5.0/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:274:in `require': no implicit conversion from nil to integer (TypeError)