Service Worker not receiving message

NOTE: Using Create-React-App Template... (Modifying the Service Worker in it)

I am using the communication channel api to send a message to a service worker for caching. I use an xmlhttp request because of its progress api since fetch needs an indefinite loader afaik.

So after receiving the data on readystate 4 and status code 200 I go to postMessage to SW. I get logging on client side but don't receive the message in the service worker.

I am developing locally and using a Chrome Extension to allow local testing of SW and Build: https://chrome.google.com/webstore/detail/web-server-for-chrome/ofhbbkphhbklhfoeikjpcbhemlocgigb?hl=en

CLIENT SNIPPET

xhr.onreadystatechange = function(){
    if (this.readyState == 4 && this.status == 200) {
        const res = JSON.parse(this.responseText);

        function sendMessage(msg){
            console.log("SEND SW MESSAGE");
            const msgChan = new MessageChannel();
            // This wraps the message posting/response in a promise, which will resolve if the response doesn't
            // contain an error, and reject with the error if it does. If you'd prefer, it's possible to call
            // controller.postMessage() and set up the onmessage handler independently of a promise, but this is
            // a convenient wrapper.
            return new Promise(function(resolve, reject){
                console.log("Promise Scope");

                msgChan.port1.onmessage = function(event){
                    event.data.error ? reject(event.data.error) : resolve(event.data);
                }
                // This sends the message data as well as transferring messageChannel.port2 to the service worker.
                // The service worker can then use the transferred port to reply via postMessage(), which
                // will in turn trigger the onmessage handler on messageChannel.port1.
                // See https://html.spec.whatwg.org/multipage/workers.html#dom-worker-postmessage
                navigator.serviceWorker.controller.postMessage(msg, [msgChan.port2]);
            });
        }

        sendMessage(res).then(function(){
            console.log("SW MESSAGE SENT");
            // Storing Quake Data
            that.props.setQuakes(res[0]);
            // Storing Three Data Obj - SSR Parsing and Creation
            that.props.setThreeData(res[1]);   
            // Greenlight 
            that.props.setVizInitSuccess(true);
        }).catch(function(err){
            console.log("Error Caching Data: "+err);
        });
    } };

SERVICE WORKER SNIPPET

function registerValidSW(swUrl, config) {
  navigator.serviceWorker
    .register(swUrl)
    .then(registration => {
      // Set up a listener for messages posted from the service worker.
      // The service worker is set to post a message to all its clients once it's run its activation
      // handler and taken control of the page, so you should see this message event fire once.
      // You can force it to fire again by visiting this page in an Incognito window.
      navigator.serviceWorker.onmessage = function(event) {
        console.log("SERVICE WORKER RECIEVED MESSAGE");
        console.log(event);

        event.ports[0].postMessage("SW Says Hello Back!");
        if (event.data.requireData == true && 'caches' in window) {
          // Check for cache'd data and load
          // clients.matchAll().then(clients => {
          //     clients.forEach(client => {
          //         console.log(client);
          //         //send_message_to_client(client, msg).then(m => console.log("SW Received Message: "+m));
          //     })
          // })

          // caches.open('threeData').then(function(cache){
          //   console.log("SW Cache");
          //   console.log(cache)
          //   event.ports[0].postMessage(cache);  
          // });

        } else {
          // Cache Data
          caches.open('threeData').then(function(cache){
            cache.put('/data.json', new Response(event.data.json))
          });
        }
      };
...

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