Not able to subscribe the PushManager
<p>I am nww to GAE and push API/service workers, I am trying to subscribe the pushManager but, pushManager getSubscription method handler returns the null reference </p>

<pre><code>navigator.serviceWorker.ready.then(function(serviceWorkerRegistration)
 {      console.log('in side ready ' );
     // Do we already have a push message subscription?  
     serviceWorkerRegistration.pushManager.getSubscription()  
       .then(function(subscription) {  

         // Enable any UI which subscribes / unsubscribes from  
         // push messages.  
         var pushButton = document.querySelector('.js-push-button');  
         pushButton.disabled = false;

         if (!subscription) {  
           // We aren't subscribed to push, so set UI  
           // to allow the user to enable push        console.log('subscription error ' ); 
           return;  
         }  console.log('subscriptioned ' );
         // Keep your server in sync with the latest subscriptionId
         sendSubscriptionToServer(subscription);

         // Set your UI to show they have subscribed for  
         // push messages  
         pushButton.textContent = 'Disable Push Messages';  
         isPushEnabled = true;  
       })  
       .catch(function(err) {  
         console.warn('Error during getSubscription()', err);  
       });     });
</code></pre>

<p>In able code <code>getSubscription()</code> then returns subscription value as null, so that, control comes to if block and function is existing</p>

<p>if i am trying to call the </p>

<pre><code>serviceWorkerRegistration.pushManager.subscribe() 
</code></pre>

<p>then I am getting the below error</p>

<pre><code>registration failed - no sender id provided
</code></pre>

<p>Please find the working progress url below 
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