avoid error on browsers without a service worker

src/App.vue

@@ -353,7 +353,7 @@ export default class App extends Vue {
       }
     }
     // there may be a long pause here on first initialization
-    navigator.serviceWorker.ready.then(() => {
+    navigator.serviceWorker?.ready.then(() => {
       this.serviceWorkerReady = true;
     });
   }
@@ -443,7 +443,7 @@ export default class App extends Vue {
         this.subscribeToPush()
           .then(() => {
             console.log("Subscribed successfully.");
-            return navigator.serviceWorker.ready;
+            return navigator.serviceWorker?.ready;
           })
           .then((registration) => {
             return registration.pushManager.getSubscription();
@@ -575,7 +575,7 @@ export default class App extends Vue {
 
   async turnOffNotifications() {
     let subscription;
-    const pushProviderSuccess = await navigator.serviceWorker.ready
+    const pushProviderSuccess = await navigator.serviceWorker?.ready
       .then((registration) => {
         return registration.pushManager.getSubscription();
       })