fix(active_job): remove nil from global serializers set during app re-initialization

Author: solnic

sentry-rails/spec/dummy/test_rails_app/config/application.rb

@@ -153,6 +153,20 @@ def after_initialize!
         end
 
         def cleanup!
+          # Re-initialising the app triggers the active_job.custom_serializers
+          # initializer which calls add_serializers(app.config.active_job.custom_serializers).
+          # Under some Rails/Ruby combinations custom_serializers resolves to nil instead
+          # of the railtie default of [], inserting nil into the global serializers Set.
+          # Remove it here so it does not outlive this app instance.
+          # Rails < 8 uses a mattr_accessor _additional_serializers; Rails 8+ uses @serializers.
+          if defined?(::ActiveJob::Serializers)
+            if ::ActiveJob::Serializers.respond_to?(:_additional_serializers)
+              ::ActiveJob::Serializers._additional_serializers.delete(nil)
+            elsif ::ActiveJob::Serializers.instance_variable_defined?(:@serializers)
+              ::ActiveJob::Serializers.instance_variable_get(:@serializers).delete(nil)
+            end
+          end
+
           ::Rails.application = nil
         end