Webpackerを使用したRailsアプリケーションのHerokuデプロイが失敗する

今月初めにデプロイしたときは大丈夫だったのに。

```
remote:        [1/4] Resolving packages...
remote:        [2/4] Fetching packages...
remote:        info fsevents@1.2.9: The platform "linux" is incompatible with this module.
remote:        info "fsevents@1.2.9" is an optional dependency and failed compatibility check. Excluding it from installation.
remote:        [3/4] Linking dependencies...
remote:        warning " > webpack-dev-server@3.5.1" has unmet peer dependency "webpack@^4.0.0".
remote:        warning "webpack-dev-server > webpack-dev-middleware@3.7.0" has unmet peer dependency "webpack@^4.0.0".
remote:        [4/4] Building fresh packages...
remote:        Done in 23.46s.
remote:        rake aborted!
remote:        NoMethodError: undefined method `+' for nil:NilClass
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/configuration.rb:31:in `additional_paths'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/configuration.rb:35:in `additional_paths_globbed'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:93:in `default_watched_paths'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:57:in `watched_files_digest'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:38:in `fresh?'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:43:in `stale?'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:22:in `compile'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/commands.rb:51:in `compile'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker.rb:35:in `compile'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:23:in `block (4 levels) in <top (required)>'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker.rb:28:in `ensure_log_goes_to_stdout'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:22:in `block (3 levels) in <top (required)>'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker.rb:20:in `with_node_env'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:21:in `block (2 levels) in <top (required)>'
remote:        /tmp/build_4930254058220dd8fc6c979c60005fdd/vendor/bundle/ruby/2.5.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
remote:        Tasks: TOP => assets:precompile => webpacker:compile
remote:        (See full trace by running task with --trace)
remote:
remote:  !
remote:  !     Precompiling assets failed.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote:
remote:  !     Push failed
remote: Verifying deploy...

```

Webpackerの元ソースに変更があったみたい。

https://github.com/rails/webpacker/commit/0adb3dc342addaa00122f9b022bf05606225c39c#diff-04c6e90faac2675aa89e2176d2eec7d8R365

```
- with webpack modules, you can use the `resolved_paths`
+ with webpack modules, you can use the `additional_paths`
```
のとおり、webpacker.ymlのresolved_pathsをadditional_pathsに変更して通った。
全くソースは読み込めてないですが取り急ぎ。