window.location.assign and <a href''> stop triggering fetch after SW registration and Firebase Auth login

### Introduced Code Changes:
I replaced the current client-side `getIdToken()` method with the following block that registers an onIdTokenChanged listener to independently refresh the token inside SW when it's changed or user is logged out without the need to retrieve the token every time a request is sent:
```javascript
// CurrentUser module, provides up-to-date idToken
const CurrentUser = (() => {
let IDToken = '';

firebase.auth().onIdTokenChanged((user) => {
if(user) {
user.getIdToken()
.then(idToken => IDToken = idToken)
.catch(error => console.log(error));
} else {
IDToken = '';
}
});
// Public method
return {
getIdToken: () => { return Promise.resolve(IDToken); }
};
})();
```
I also then changed the `event.respondWith()` to this since the `.getIdToken()` always resolves and `.then()` no longer needs second argument for onRejected event
```javascript
event.respondWith(CurrentUser.getIdToken().then(requestProcessor));
```
### Observed Issue:
When registering the ServiceWorker, after a user logs in with Firebase Auth, `window.location.assign()` and anchor tag navigation `` fail to cause FetchEvent event. However, manual change of URL results in FetchEvent and proper navigation (token injection also takes place correctly).

## Any thoughts what may be causing this or how to fix it?