Unauthorized connection attempt was rejected with Rails 5 ActionCable

Following the hartle tutorial here: https://www.learnenough.com/action-cable-tutorial#sec-upgrading_to_action_cable

When I get to Step 4, adding ActionCable the chat messages are not transmitted and I get the error:

User Load (0.4ms)  SELECT  "users".* FROM "users" WHERE "users"."id" IS NULL LIMIT ?  [["LIMIT", 1]]
An unauthorized connection attempt was rejected

here are the relevant files:

room_channel.rb:

class RoomChannel < ApplicationCable::Channel
  def subscribed
    stream_from "room_channel"
  end

  def unsubscribed
    # Any cleanup needed when channel is unsubscribed
  end
end

messages controller:

  class MessagesController < ApplicationController
  before_action :logged_in_user
  before_action :get_messages

  def index
  end

  def create
    message = current_user.messages.build(message_params)
    if message.save
      ActionCable.server.broadcast 'room_channel',
                               message: render_message(message)
      message.mentions.each do |mention|
        ActionCable.server.broadcast "room_channel_user_#    {mention.id}",
                                     mention: true
      end
    end
  end

  private

    def get_messages
      @messages = Message.for_display
      @message  = current_user.messages.build
    end

    def message_params
      params.require(:message).permit(:content)
    end

    def render_message(message)
      render(partial: 'message', locals: { message: message })
    end
end

room.coffee:

App.room = App.cable.subscriptions.create "RoomChannel",
  connected: ->
    # Called when the subscription is ready for use on the server

  disconnected: ->
    # Called when the subscription has been terminated by the server

  received: (data) ->
    # Called when there's incoming data on the websocket for this     channel
    alert data.content

routes.rb:

Rails.application.routes.draw do
  root 'messages#index'
  resources :users
  resources :messages
  get    '/login',   to: 'sessions#new'
  post   '/login',   to: 'sessions#create'
  delete '/logout',  to: 'sessions#destroy'

  mount ActionCable.server, at: '/cable'
end 

The reference branch works fine on my machine, but I can't get my tutorial branch to use AC.

Update:

Skipping down to Section 5 of the tutorial, I added connection.rb, which had been blank in the tutorial's beginning repo as follows:

connection.rb:

module ApplicationCable
  class Connection < ActionCable::Connection::Base
    include SessionsHelper

    identified_by :message_user

    def connect
      self.message_user = find_verified_user
    end

    private

      def find_verified_user
        if logged_in?
          current_user
        else
          reject_unauthorized_connection
        end
      end
  end
end

And broadcasting seems to work in one direction. I have two tabs open. but only one works to broadcast messages. In the other, the console shows this error:

Error: Existing connection must be closed before opening action_cable.self-17ebe4af84895fa064a951f57476799066237d7bb5dc4dc351a8b01cca19cce9.js:231:19
Connection.prototype.open
http://localhost:3000/assets/action_cable.self-17ebe4af84895fa064a951f57476799066237d7bb5dc4dc351a8b01cca19cce9.js:231:19
bind/<
http://localhost:3000/assets/action_cable.self-17ebe4af84895fa064a951f57476799066237d7bb5dc4dc351a8b01cca19cce9.js:201:60

In the logs, with the above connection.rb, the search for null user is gone, showing this:

User Load (0.3ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
Registered connection (Z2lkOi8vY2hhdC1hcHAvVXNlci8x)
RoomChannel is transmitting the subscription confirmation
RoomChannel is streaming from room_channel
Started GET "/cable" for ::1 at 2018-12-29 08:04:31 -0500
Started GET "/cable/" [WebSocket] for ::1 at 2018-12-29 08:04:31 -0500
Successfully upgraded to WebSocket (REQUEST_METHOD: GET, HTTP_CONNECTION: keep-alive, Upgrade, HTTP_UPGRADE: websocket)