rails Herokuにデブロイエラー「NoMethodError: undefined method `+' for nil:NilClass」が解決できない。

herokuにstaging環境を作り、デプロイしようとすると以下のエラーが出て成功しません。

ruby
1remote:        yarn install v1.16.0
2remote:        [1/4] Resolving packages...
3remote:        [2/4] Fetching packages...
4remote:        info fsevents@1.2.12: The platform "linux" is incompatible with this module.
5remote:        info "fsevents@1.2.12" is an optional dependency and failed compatibility check. Excluding it from installation.
6remote:        [3/4] Linking dependencies...
7remote:        warning " > webpack-dev-server@3.10.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
8remote:        warning "webpack-dev-server > webpack-dev-middleware@3.7.2" has unmet peer dependency "webpack@^4.0.0".
9remote:        [4/4] Building fresh packages...
10remote:        Done in 26.43s.
11remote:        I, [2020-05-12T14:30:21.926517 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/application-acbbcf2b20b4733f9a58938d54f55206396bc493119f5d088ef0ecda5ccf9d82.js
12remote:        I, [2020-05-12T14:30:21.926791 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/application-acbbcf2b20b4733f9a58938d54f55206396bc493119f5d088ef0ecda5ccf9d82.js.gz
13remote:        I, [2020-05-12T14:30:21.928843 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/actiontext-8f4769a67d984eef16e806b8c7796898df4a231778524cbcc58938caa95ef1f3.css
14remote:        I, [2020-05-12T14:30:21.929484 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/actiontext-8f4769a67d984eef16e806b8c7796898df4a231778524cbcc58938caa95ef1f3.css.gz
15remote:        I, [2020-05-12T14:30:21.936307 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/application-cee0c5739406c64710f21ed0b4aff6d7a375d5673d9eaa6bdfa35b542273c921.css
16remote:        I, [2020-05-12T14:30:21.936511 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/application-cee0c5739406c64710f21ed0b4aff6d7a375d5673d9eaa6bdfa35b542273c921.css.gz
17remote:        I, [2020-05-12T14:30:21.980031 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/blog-495deb931acf2cb74325ce78a5e5f2ce96ffc5de2716ec1a14d4fc9eca31608e.css
18remote:        I, [2020-05-12T14:30:21.980678 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/blog-495deb931acf2cb74325ce78a5e5f2ce96ffc5de2716ec1a14d4fc9eca31608e.css.gz
19remote:        I, [2020-05-12T14:30:21.982207 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/mypage-fe56b933f1fae3c0cdc309e5d7b46d19d793fb24920854a91517f7753116ff6e.css
20remote:        I, [2020-05-12T14:30:21.982694 #773]  INFO -- : Writing /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/public/assets/mypage-fe56b933f1fae3c0cdc309e5d7b46d19d793fb24920854a91517f7753116ff6e.css.gz
21remote:        rake aborted!
22remote:        NoMethodError: undefined method `+' for nil:NilClass
23remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/configuration.rb:31:in `additional_paths'
24remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/configuration.rb:35:in `additional_paths_globbed'
25remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:93:in `default_watched_paths'
26remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:57:in `watched_files_digest'
27remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:38:in `fresh?'
28remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:43:in `stale?'
29remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/compiler.rb:22:in `compile'
30remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker/commands.rb:51:in `compile'
31remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker.rb:35:in `compile'
32remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:23:in `block (4 levels) in <main>'
33remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker.rb:28:in `ensure_log_goes_to_stdout'
34remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:22:in `block (3 levels) in <main>'
35remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/webpacker.rb:20:in `with_node_env'
36remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:21:in `block (2 levels) in <main>'
37remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/bundler/gems/webpacker-22ab02b7c84e/lib/tasks/webpacker/compile.rake:14:in `block in enhance_assets_precompile'
38remote:        /tmp/build_e7fcd7f45c9309a1194c26c5778da9b3/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
39remote:        Tasks: TOP => webpacker:compile
40remote:        (See full trace by running task with --trace)
41remote: 
42remote:  !
43remote:  !     Precompiling assets failed.
44remote:  !
45remote:  !     Push rejected, failed to compile Ruby app.
46remote: 
47remote:  !     Push failed
48remote: Verifying deploy...
49

「NoMethodError: undefined method `+' for nil:NilClass」とあるので、プロジェクト内を'+'の記述ミスがないか探しましたが、見つけることができず、
同じコードでproduction環境へはデプロイできるがstaging環境のみデプロイに失敗してしまうので、webpacker.ymlあたりが間違っているかもしれませんが、それを見つけられずにいます。

ruby
1default: &default
2  source_path: app/javascript
3  source_entry_path: packs
4  public_root_path: public
5  public_output_path: assets
6  cache_path: tmp/cache/webpacker
7  webpack_compile_output: true
8
9  resolved_paths: []
10
11  cache_manifest: false
12
13  extract_css: false
14
15  static_assets_extensions:
16    - .jpg
17    - .jpeg
18    - .png
19    - .gif
20    - .tiff
21    - .ico
22    - .svg
23    - .eot
24    - .otf
25    - .ttf
26    - .woff
27    - .woff2
28
29  extensions:
30    - .mjs
31    - .js
32    - .sass
33    - .scss
34    - .css
35    - .module.sass
36    - .module.scss
37    - .module.css
38    - .png
39    - .svg
40    - .gif
41    - .jpeg
42    - .jpg
43
44development:
45  <<: *default
46  compile: true
47
48  # Reference: https://webpack.js.org/configuration/dev-server/
49  dev_server:
50    https: false
51    host: localhost
52    port: 3035
53    public: localhost:3035
54    hmr: false
55    # Inline should be set to true if using HMR
56    inline: true
57    overlay: true
58    compress: true
59    disable_host_check: true
60    use_local_ip: false
61    quiet: false
62    pretty: false
63    headers:
64      'Access-Control-Allow-Origin': '*'
65    watch_options:
66      ignored: '**/node_modules/**'
67
68
69test:
70  <<: *default
71  compile: true
72
73  # Compile test packs to a separate directory
74  public_output_path: packs-test
75
76production:
77  <<: *default
78
79  compile: false
80
81  extract_css: true
82
83  cache_manifest: true
84
85staging:
86  <<: *default
87
88  compile: false
89
90  cache_manifest: true
91
92  public_output_path: packs-staging
93
94  public_root_path: public