Cannot read property 'pushManager' of null

the code i used is

'use strict';
const applicationServerPublicKey = 'BNrC14_SrPGHLHhy2c_c5Q-PPTqFHyPJBdshSPVPWABQokwR6oA6Bok8G00tBsn50u6VtgHiQd8i5uszTi4xrfY';

  const pushButton = document.querySelector('.js-push-btn');

  let isSubscribed = false;
  let swRegistration = null;

  function urlB64ToUint8Array(base64String) {
    const padding = '='.repeat((4 - base64String.length % 4) % 4);
    const base64 = (base64String + padding)
      .replace(/\-/g, '+')
      .replace(/_/g, '/');

    const rawData = window.atob(base64);
    const outputArray = new Uint8Array(rawData.length);

    for (let i = 0; i < rawData.length; ++i) {
      outputArray[i] = rawData.charCodeAt(i);
    }
    return outputArray;
  }
  if ('serviceWorker' in navigator && 'PushManager' in window) {
    console.log('Service Worker and Push is supported');

    navigator.serviceWorker.register('sw.js')
    .then(function(swReg) {
      console.log('Service Worker is registered', swReg);

      swRegistration = swReg;
    })
    .catch(function(error) {
      console.error('Service Worker Error', error);
    });
  } else {
    console.warn('Push messaging is not supported');
    pushButton.textContent = 'Push Not Supported';
  }



  function initializeUI() {
    // Set the initial subscription value
    swRegistration.pushManager.getSubscription()
    .then(function(subscription) {
      isSubscribed = !(subscription === null);

      if (isSubscribed) {
        console.log('User IS subscribed.');
      } else {
        console.log('User is NOT subscribed.');
      }

      updateBtn();
    });
  }

  function updateBtn() {
    if (isSubscribed) {
      pushButton.textContent = 'Disable Push Messaging';
    } else {
      pushButton.textContent = 'Enable Push Messaging';
    }

    pushButton.disabled = false;
  }

    navigator.serviceWorker.register('sw.js')
      .then(function(swReg) {
    console.log('Service Worker is registered', swReg);

    swRegistration = swReg;
      initializeUI();
    })

  function initializeUI() {
    pushButton.addEventListener('click', function() {
      pushButton.disabled = true;
      if (isSubscribed) {
        // TODO: Unsubscribe user
      } else {
        subscribeUser();
      }
    });
    // Set the initial subscription value
    swRegistration.pushManager.getSubscription()
    .then(function(subscription) {
      isSubscribed = !(subscription === null);

      updateSubscriptionOnServer(subscription);

      if (isSubscribed) {
        console.log('User IS subscribed.');
      } else {
        console.log('User is NOT subscribed.');
      }

      updateBtn();
    });
  }

  function subscribeUser() {
    const applicationServerKey = urlB64ToUint8Array(applicationServerPublicKey);
    swRegistration.pushManager.subscribe({
      userVisibleOnly: true,
      applicationServerKey: applicationServerKey
    })
    .then(function(subscription) {
      console.log('User is subscribed.');

      updateSubscriptionOnServer(subscription);

      isSubscribed = true;

      updateBtn();
    })
    .catch(function(err) {
      console.log('Failed to subscribe the user: ', err);
      updateBtn();
    });
  }

  const applicationServerKey = urlB64ToUint8Array(applicationServerPublicKey);
  swRegistration.pushManager.subscribe({
    userVisibleOnly: true,
    applicationServerKey: applicationServerKey
  })
  swRegistration.pushManager.subscribe({
    userVisibleOnly: true,
    applicationServerKey: applicationServerKey
  })
  .then(function(subscription) {
    console.log('User is subscribed.');

    updateSubscriptionOnServer(subscription);

    isSubscribed = true;

    updateBtn();

  })
  .catch(function(err) {
    console.log('Failed to subscribe the user: ', err);
    updateBtn();
  });

  function updateSubscriptionOnServer(subscription) {
    // TODO: Send subscription to application server
    const subscriptionJson = document.querySelector('.js-subscription-json');
    const subscriptionDetails =
      document.querySelector('.js-subscription-details');

    if (subscription) {
      subscriptionJson.textContent = JSON.stringify(subscription);
      subscriptionDetails.classList.remove('is-invisible');
    } else {
      subscriptionDetails.classList.add('is-invisible');
    }
  }
  function updateBtn() {
    if (Notification.permission === 'denied') {
      pushButton.textContent = 'Push Messaging Blocked.';
      pushButton.disabled = true;
      updateSubscriptionOnServer(null);
      return;
    }

    if (isSubscribed) {
      pushButton.textContent = 'Disable Push Messaging';
    } else {
      pushButton.textContent = 'Enable Push Messaging';
    }

    pushButton.disabled = false;
  }

  self.addEventListener('push', function(event) {
    console.log('[Service Worker] Push Received.');
    console.log(`[Service Worker] Push had this data: "${event.data.text()}"`);

    const title = 'Push Codelab';
    const options = {
      body: 'Yay it works.',
      icon: 'images/icon.png',
      badge: 'images/badge.png'
    };
    const notificationPromise = self.registration.showNotification(title, options);
    event.waitUntil(notificationPromise);
    
  });


  self.addEventListener('notificationclick', function(event) {
    console.log('[Service Worker] Notification click Received.');

    event.notification.close();

    event.waitUntil(
      clients.openWindow('https://developers.google.com/web/')
    );
  });

hey,
after following the steps on Docs
unfortunately i can't understand why pushManager is not working
i tried to click push on webDev but nothing happened.

the User is already subscribed.
thanks

[milanmeu]

Hi Mr4FX

Main problem

swRegistration is defined on line 7 with value null. On line 124 you try to read pushManager from swRegistration. This is not possible because there is no property "pushManager" in null (swRegistration).
swRegistration will be changed on line 30 to swReg but line 124 has been executed earlier.
Lines 123 to 145 must be placed inside a function for it to be executed when a button is pressed. The code will not be executed / the button will be unavailable until the service worker is registered.
However, these lines are already placed in a function on lines 103 to 121. So your code is a duplicate here. It is enough to delete lines 123 to 145.
The code should now work.

Tips

• Lines 177 to 201 must be placed in the sw.js file so that these functions can be performed even when the site is not opened.
• Change the indentation of your code, there are currently too many spaces. This makes the code difficult to read for us and yourself.
• You have not posted the first part of your code. This is probably copyright information. We do not need that, but now it's difficult to find the correct line that is mentioned in the error message in the console.

I hope I have been able to help you. Let me know if you have any questions.