what is a boilerplate way to define rtm?

I am trying to test my new Slackbot by sending it a greeting with the expected behavior that it replies with 'this is a test message' in accordance with my code here in slackClient.js:

'use strict'

const RtmClient = require('@slack/client').RtmClient;
const CLIENT_EVENTS = require('@slack/client').CLIENT_EVENTS;
const RTM_EVENTS = require('@slack/client').RTM_EVENTS;
let rtm = null;

function handleOnAuthenticated(rtmStartData) {
  console.log(`Logged in as ${rtmStartData.self.name} of team ${rtmStartData.team.name}, but not yet connected to a channel`);
}

function handleOnMessage(message) {
  console.log(message);
  // This will send the message 'this is a test message' to the channel identified by id 'C0CHZA86Q'
  rtm.sendMessage('this is a test message', message.channel, function messageSent() {
    // optionally, you can supply a callback to execute once the message has been sent
  });
}

function addAuthenticatedHandler(rtm, handler) {
  rtm.on(CLIENT_EVENTS.RTM.AUTHENTICATED, handler);
}

module.exports.init = function slackClient(bot_token, logLevel){
  const rtm = new RtmClient(bot_token);
  addAuthenticatedHandler(rtm, handleOnAuthenticated);
  rtm.on(RTM_EVENTS.MESSAGE, handleOnMessage)
  return rtm;
}

module.exports.addAuthenticatedHandler = addAuthenticatedHandler;

This is the error I get:

 rtm.sendMessage('this is a test message', message.channel, function messageSent() {
     ^

TypeError: Cannot read property 'sendMessage' of null

If I understand this correctly, it's telling me I cannot give rtm a value of null, but then what kind of boilerplate value can I give it just so that I can test this out?

via Daniel