Problem with Rails when upgrading version - ruby-on-rails-5

Question

We have a webap that was initially born in 2.5 ruby.
We have upgraded to 2.6 and then, we have a problem to serve its pages.
Here is the error we receive:
NoMethodError in SingleviewController#index
undefined method `[]' for nil:NilClass
Extracted source (around line #36):
34
35
36
37
38
39
end
unless headers[CACHE_CONTROL]
if digest
headers[CACHE_CONTROL] = #cache_control if #cache_control
else
Application Trace | Framework Trace | Full Trace
rack (2.0.8) lib/rack/etag.rb:36:in `call'
rack (2.0.8) lib/rack/conditional_get.rb:25:in `call'
rack (2.0.8) lib/rack/head.rb:12:in `call'
actionpack (6.0.2.2) lib/action_dispatch/http/content_security_policy.rb:18:in `call'
rack (2.0.8) lib/rack/session/abstract/id.rb:259:in `context'
rack (2.0.8) lib/rack/session/abstract/id.rb:253:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/cookies.rb:648:in `call'
activerecord (6.0.2.2) lib/active_record/migration.rb:567:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/callbacks.rb:27:in `block in call'
activesupport (6.0.2.2) lib/active_support/callbacks.rb:101:in `run_callbacks'
actionpack (6.0.2.2) lib/action_dispatch/middleware/callbacks.rb:26:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/actionable_exceptions.rb:17:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/debug_exceptions.rb:32:in `call'
web-console (3.7.0) lib/web_console/middleware.rb:135:in `call_app'
web-console (3.7.0) lib/web_console/middleware.rb:30:in `block in call'
web-console (3.7.0) lib/web_console/middleware.rb:20:in `catch'
web-console (3.7.0) lib/web_console/middleware.rb:20:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
railties (6.0.2.2) lib/rails/rack/logger.rb:38:in `call_app'
railties (6.0.2.2) lib/rails/rack/logger.rb:26:in `block in call'
activesupport (6.0.2.2) lib/active_support/tagged_logging.rb:80:in `block in tagged'
activesupport (6.0.2.2) lib/active_support/tagged_logging.rb:28:in `tagged'
activesupport (6.0.2.2) lib/active_support/tagged_logging.rb:80:in `tagged'
railties (6.0.2.2) lib/rails/rack/logger.rb:26:in `call'
sprockets-rails (3.2.1) lib/sprockets/rails/quiet_assets.rb:13:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/request_id.rb:27:in `call'
rack (2.0.8) lib/rack/method_override.rb:22:in `call'
rack (2.0.8) lib/rack/runtime.rb:22:in `call'
activesupport (6.0.2.2) lib/active_support/cache/strategy/local_cache_middleware.rb:29:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/executor.rb:14:in `call'
rack-livereload (0.3.17) lib/rack/livereload.rb:23:in `_call'
rack-livereload (0.3.17) lib/rack/livereload.rb:14:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/static.rb:126:in `call'
rack (2.0.8) lib/rack/sendfile.rb:111:in `call'
actionpack (6.0.2.2) lib/action_dispatch/middleware/host_authorization.rb:83:in `call'
webpacker (4.2.2) lib/webpacker/dev_server_proxy.rb:23:in `perform_request'
rack-proxy (0.6.5) lib/rack/proxy.rb:57:in `call'
railties (6.0.2.2) lib/rails/engine.rb:526:in `call'
puma (4.3.5) lib/puma/configuration.rb:228:in `call'
puma (4.3.5) lib/puma/server.rb:713:in `handle_request'
puma (4.3.5) lib/puma/server.rb:472:in `process_client'
puma (4.3.5) lib/puma/server.rb:328:in `block in run'
puma (4.3.5) lib/puma/thread_pool.rb:134:in `block in spawn_thread'
I am a newby. What else do you need to find my problem?
I really don't know where to start looking at.
Best regards.

Answer 1

Based on the provided code sample, it looks like your headers method is returning nil for some reason. I would try two things:
First, change headers to request.headers
If that doesn't work, change your code to something like:
begin
unless headers[CACHE_CONTROL]
if digest
headers[CACHE_CONTROL] = #cache_control if #cache_control
else
...
rescue
# You should probably do something more meaningful here.
# This is only meant to return a rescue value for this design pattern
nil
end