title: The return type 'Transaction' isn't a 'Future<_>', as required by the closure's context
body: <pre><code>Future&lt;void&gt; init() async {
      if (!_initialized) {
        _firebaseMessaging.requestNotificationPermissions();
  
        User user = FirebaseAuth.instance.currentUser;
        _uid = user.uid;
  
        _firebaseMessaging.onTokenRefresh.listen((event) {
          var db = FirebaseFirestore.instance;
          var ref = FirebaseFirestore.instance.doc(&quot;users/$_uid&quot;);
          db.runTransaction(
              (transaction) =&gt; transaction.update(ref, {constants.fcm: event}));
        });
  </code></pre>
  <p>I'm getting a return type error in this line:</p>
  <pre><code>db.runTransaction(
                  (transaction) =&gt; transaction.update(ref, {constants.fcm: event}));
  </code></pre>
  <p><strong>The return type 'Transaction' isn't a 'Future&lt;_&gt;', as required by the closure's context</strong></p>