push subscriptionchange upload token null

problem-1-upload-token.ts

async handlePush(event: PushSubscriptionChangeEvent) {
  const req = new Request('/refreshpushsubscription', {
    method: 'POST',
    body: JSON.stringify(
        {oldSubscription: event.oldSubscription, newSubscription: event.newSubscription})
  });
  const response = await self.fetch(req);
}

self.addEventListener('pushsubscriptionchange', (event: PushSubscriptionChangeEvent) => 
                      event.waitUntil(handlePush(event)));