application.rb

# config/application.rb
module MyApp
  class Application < Rails::Application
    
    # Handle exceptions with a custom controller
    config.exceptions_app = lambda do |env|
      ErrorsController.action(:show).call(env)
    end
  end
end

error.html.erb

<%= javascript_tag do %>
document.addEventListener('DOMContentLoaded', function() {
  Raven.showReportDialog({
    eventId: '<%= @sentry_event_id %>'
  })
});
<% end %>

errors_controller.rb

class ErrorsController < ApplicationController
  before_action :fetch_exception

  layout 'error'

  def show
    # Get Sentry event for error feedback
    @sentry_event_id = Raven.last_event_id

    render "errors/#{@rescue_response}", status: @status_code
  rescue ActionView::MissingTemplate
    render 'errors/internal_server_error', status: 500
  end


  private

  # Resolves an exception to a HTTP error and status
  def fetch_exception
    @exception = request.env['action_dispatch.exception']
    @status_code = ActionDispatch::ExceptionWrapper.status_code_for_exception(@exception.class.name)
    @rescue_response = ActionDispatch::ExceptionWrapper.rescue_responses[@exception.class.name]
  end
end

https://gist.github.com/phildionne/244dffd492fd795eff3a6d2e1ff55f47#file-errors_controller-rb-L8 this is Nil