Trying to get the LMS demo working in a scratch

expSubscriber.html

<template>
  <div>
    <button onclick={subscribeMC}>Subscribe</button>
  </div>
  <div>
    <button onclick={unsubscribeMC}>Unsubscribe</button>
  </div>
</template>

expSubscriber.js

import { LightningElement, wire } from 'lwc'
import { APPLICATION_SCOPE, subscribe, unsubscribe, MessageContext } from 'lightning/messageService'
import SAMPLEMC from '@salesforce/messageChannel/SampleMessageChannel__c'

export default class ExpSubscriber extends LightningElement {
  @wire(MessageContext)
  messageContext

  subscription = null

  subscribeMC () {
    if (this.subscription) return

    console.log('begin sub to', this.messageContext)

    this.subscription = subscribe(
      this.messageContext,
      SAMPLEMC,
      this.handleMessage,
      { scope: APPLICATION_SCOPE })

    console.log('subscribed', this.subscription)
  }

  unsubscribeMC () {
    console.log('begin unsub', this.subscription)
    unsubscribe(this.subscription)
    this.subscription = null

    console.log('unsubscribed', this.subscription)
  }

  handleMessage (message) {
    console.log('sub message', JSON.parse(JSON.stringify(message)))
  }
}

parent-lwc.html

<template>
  <div>
    <c-exp-publisher></c-exp-publisher>

    <c-exp-subscriber></c-exp-subscriber>
  </div>
</template>