Rails Deployment to Heroku: NoMethodError: undefined method `+' for nil:NilClass - ruby-on-rails

Question

I'm new to Rails but have been working on an app the last few weeks that has been working great in development and in production when I deploy to Heroku.
Today I made some minor changes to my application.css file and now I am getting a "Precompiling assets failed" error when I try to deploy to Heroku. With a NoMethodError: undefined method `+' for nil:NilClass. Running ruby 2.6.5, Rails 6.0.3.1, PG 1.2.3
I've attached the logs below which may or may not be helpful. I obviously don't have the knowledge of how to debug this properly so advice on what might be causing the error is great but failing that, how should I be trying to replicate this error locally and figuring it out?
I've already tried, with no success:
Changing the application.css file to .scss
Adding config.assets.initialize_on_precompile = false to the application.rb file
Reverting to my last successful commit
Thank you!
Enumerating objects: 78, done.
Counting objects: 100% (78/78), done.
Delta compression using up to 8 threads
Compressing objects: 100% (61/61), done.
Writing objects: 100% (61/61), 5.07 KiB | 519.00 KiB/s, done.
Total 61 (delta 46), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Ruby app detected
remote: -----> Installing bundler 1.17.3
remote: -----> Removing BUNDLED WITH version in the Gemfile.lock
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.6.5
remote:
remote: ###### WARNING:
remote:
remote: Removing `Gemfile.lock` because it was generated on Windows.
remote: Bundler will do a full resolve so native gems are handled properly.
remote: This may result in unexpected gem versions being used in your app.
remote: In rare occasions Bundler may not be able to resolve your dependencies at all.
remote: https://devcenter.heroku.com/articles/bundler-windows-gemfile
remote:
remote: -----> Installing dependencies using bundler 1.17.3
remote: Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4
remote: The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64
-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
remote: Fetching gem metadata from https://rubygems.org/.............
remote: Fetching gem metadata from https://rubygems.org/.
remote: Resolving dependencies.....
remote: Using rake 13.0.1
remote: Using concurrent-ruby 1.1.6
remote: Using minitest 5.14.1
remote: Using thread_safe 0.3.6
remote: Using zeitwerk 2.3.0
remote: Using builder 3.2.4
remote: Using erubi 1.9.0
remote: Using mini_portile2 2.4.0
remote: Using crass 1.0.6
remote: Using rack 2.2.2
remote: Using nio4r 2.5.2
remote: Fetching websocket-extensions 0.1.5
remote: Using mimemagic 0.3.5
remote: Using mini_mime 1.0.2
remote: Using formtastic_i18n 0.6.0
remote: Using method_source 1.0.0
remote: Using thor 1.0.1
remote: Using kaminari-core 1.2.1
remote: Fetching ffi 1.13.0
remote: Using tilt 2.0.10
remote: Using bcrypt 3.1.13
remote: Using msgpack 1.3.3
remote: Using bundler 1.17.3
remote: Using orm_adapter 0.5.0
remote: Using hashids 1.0.5
remote: Using pg 1.2.3
remote: Using turbolinks-source 5.2.0
remote: Fetching i18n 1.8.3
remote: Installing websocket-extensions 0.1.5
remote: Installing i18n 1.8.3
remote: Using tzinfo 1.2.7
remote: Using nokogiri 1.10.9
remote: Using puma 4.3.5
remote: Using rack-test 1.1.0
remote: Fetching sprockets 4.0.1
remote: Installing sprockets 4.0.1
remote: Installing ffi 1.13.0 with native extensions
remote: Using warden 1.2.8
remote: Using rack-proxy 0.6.5
remote: Using marcel 0.3.3
remote: Using mail 2.7.1
remote: Using bootsnap 1.4.6
remote: Using turbolinks 5.2.1
remote: Using loofah 2.5.0
remote: Using websocket-driver 0.7.2
remote: Using activesupport 6.0.3.1
remote: Using rails-dom-testing 2.0.3
remote: Using rails-html-sanitizer 1.3.0
remote: Using globalid 0.4.2
remote: Using activemodel 6.0.3.1
remote: Using arbre 1.2.1
remote: Using jbuilder 2.10.0
remote: Using actionview 6.0.3.1
remote: Using activejob 6.0.3.1
remote: Using activerecord 6.0.3.1
remote: Using actionpack 6.0.3.1
remote: Using kaminari-actionview 1.2.1
remote: Using kaminari-activerecord 1.2.1
remote: Using polyamorous 2.3.2
remote: Using hashid-rails 1.4.0
remote: Using actioncable 6.0.3.1
remote: Using activestorage 6.0.3.1
remote: Using actionmailer 6.0.3.1
remote: Using formtastic 3.1.5
remote: Using has_scope 0.7.2
remote: Using railties 6.0.3.1
remote: Using kaminari 1.2.1
remote: Using ransack 2.3.2
remote: Using actionmailbox 6.0.3.1
remote: Using actiontext 6.0.3.1
remote: Using responders 3.0.1
remote: Using jquery-rails 4.4.0
remote: Using webpacker 4.2.2
remote: Using inherited_resources 1.11.0
remote: Using devise 4.7.1
remote: Using sprockets-rails 3.2.1
remote: Using rails 6.0.3.1
remote: Fetching sassc 2.4.0
remote: Installing sassc 2.4.0 with native extensions
remote: Using sassc-rails 2.1.2
remote: Using activeadmin 2.7.0
remote: Using sass-rails 6.0.0
remote: Bundle complete! 18 Gemfile dependencies, 77 gems now installed.
remote: Gems in the groups development and test were not installed.
remote: Bundled gems are installed into `./vendor/bundle`
remote: Post-install message from i18n:
remote:
remote: HEADS UP! i18n 1.1 changed fallbacks to exclude default locale.
remote: But that may break your application.
remote:
remote: If you are upgrading your Rails application from an older version of Rails:
remote:
remote: Please check your Rails app for 'config.i18n.fallbacks = true'.
remote: If you're using I18n (>= 1.1.0) and Rails (< 5.2.2), this should be
remote: 'config.i18n.fallbacks = [I18n.default_locale]'.
remote: If not, fallbacks will be broken in your app by I18n 1.1.x.
remote:
remote: If you are starting a NEW Rails application, you can ignore this notice.
remote:
remote: For more info see:
remote: https://github.com/svenfuchs/i18n/releases/tag/v1.1.0
remote:
remote: Removing i18n (1.8.2)
remote: Removing sprockets (4.0.0)
remote: Removing websocket-extensions (0.1.4)
remote: Removing ffi (1.12.2)
remote: Removing sassc (2.3.0)
remote: Bundle completed (186.51s)
remote: Cleaning up the bundler cache.
remote: The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64
-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
remote: -----> Installing node-v10.15.3-linux-x64
remote: -----> Installing yarn-v1.16.0
remote: -----> Detecting rake tasks
remote: -----> Preparing app for Rails asset pipeline
remote: Running: rake assets:precompile
remote: yarn install v1.16.0
remote: [1/4] Resolving packages...
remote: [2/4] Fetching packages...
remote: info fsevents#1.2.13: The platform "linux" is incompatible with this module.
remote: info "fsevents#1.2.13" is an optional dependency and failed compatibility check. Excluding it from installation.
remote: [3/4] Linking dependencies...
remote: warning " > webpack-dev-server#3.11.0" has unmet peer dependency "webpack#^4.0.0 || ^5.0.0".
remote: warning "webpack-dev-server > webpack-dev-middleware#3.7.2" has unmet peer dependency "webpack#^4.0.0".
remote: [4/4] Building fresh packages...
remote: Done in 23.47s.
remote: yarn install v1.16.0
remote: [1/4] Resolving packages...
remote: [2/4] Fetching packages...
remote: info fsevents#1.2.13: The platform "linux" is incompatible with this module.
remote: info "fsevents#1.2.13" is an optional dependency and failed compatibility check. Excluding it from installation.
remote: [3/4] Linking dependencies...
remote: warning " > webpack-dev-server#3.11.0" has unmet peer dependency "webpack#^4.0.0 || ^5.0.0".
remote: warning "webpack-dev-server > webpack-dev-middleware#3.7.2" has unmet peer dependency "webpack#^4.0.0".
remote: [4/4] Building fresh packages...
remote: Done in 5.05s.
remote: I, [2020-06-05T05:23:48.123557 #791] INFO -- : Writing /tmp/build_89a8346fedd1ed19bb5329c741888493/public/assets/active_admin-312e30985f3b0561c5db85eaba9b50b1d2c6fcbcff0e0cd5ae0862f13ef86223
796473.css
remote: I, [2020-06-05T05:23:48.123844 #791] INFO -- : Writing /tmp/build_89a8346fedd1ed19bb5329c741888493/public/assets/active_admin-312e30985f3b0561c5db85eaba9b50b1d2c6fcbcff0e0cd5ae0862f13ef86223
796473.css.gz
remote: I, [2020-06-05T05:23:48.124090 #791] INFO -- : Writing /tmp/build_89a8346fedd1ed19bb5329c741888493/public/assets/active_admin/print-312e30cf99cbf27fb92065a9eacd28e6fad48fa72cadaf6e0918790e8d
35b11d8f99f6.css
remote: I, [2020-06-05T05:23:48.124270 #791] INFO -- : Writing /tmp/build_89a8346fedd1ed19bb5329c741888493/public/assets/active_admin/print-312e30cf99cbf27fb92065a9eacd28e6fad48fa72cadaf6e0918790e8d
35b11d8f99f6.css.gz
remote: I, [2020-06-05T05:23:48.124504 #791] INFO -- : Writing /tmp/build_89a8346fedd1ed19bb5329c741888493/public/assets/manifest-312e3075a11da44c802486bc6f65640aa48a730f0f684c5c07a42ba3cd1735eb3fb0
70.js
remote: I, [2020-06-05T05:23:48.124686 #791] INFO -- : Writing /tmp/build_89a8346fedd1ed19bb5329c741888493/public/assets/manifest-312e3075a11da44c802486bc6f65640aa48a730f0f684c5c07a42ba3cd1735eb3fb0
70.js.gz
remote: rake aborted!
remote: NoMethodError: undefined method `+' for nil:NilClass
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/asset.rb:138:in `etag'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/asset.rb:67:in `block in digest_path'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/asset.rb:67:in `sub'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/asset.rb:67:in `digest_path'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/manifest.rb:175:in `block in compile'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/manifest.rb:173:in `each'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/sprockets/manifest.rb:173:in `compile'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:68:in `block (3 levels) in define'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.1/lib/rake/sprocketstask.rb:148:in `with_logger'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:67:in `block (2 levels) in define'
remote: /tmp/build_89a8346fedd1ed19bb5329c741888493/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
remote: Tasks: TOP => assets:precompile
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...
remote:
remote: ! Push rejected to ...
remote:

Answer 1

Here is possible solution: not tried yet, but you should take a look.
https://github.com/rails/sprockets/issues/683
Basically in config/initializers/assets.rb, you need to update the Rails.application.config.assets.version value as well. I dont know how this affects deployment (how to know when to change this value? do we rely on deployment to fail first, then change this?)