check if service worker exists on navigator

src/main.js

@@ -62,6 +62,7 @@ for(const route of routes){
 }
 
 // listen for postMessage events from service worker
+if(navigator.serviceWorker){
     navigator.serviceWorker.addEventListener("message", async (event) => {
         if(event.data.type === "notificationclick"){
 
@@ -76,6 +77,7 @@ navigator.serviceWorker.addEventListener("message", async (event) => {
 
         }
     });
+}
 
 // access Connection class from window
 // this allows typing "Connection.log = true" in Chrome dev tools