Expo Push Notification token undefined

Im using Expo example to get the token for the device:

async function registerForPushNotificationsAsync() {
        let token;
        if (Constants.isDevice) {
          const { status: existingStatus } = await Notifications.getPermissionsAsync();
          let finalStatus = existingStatus;
          if (existingStatus !== 'granted') {
            const { status } = await Notifications.requestPermissionsAsync();
            finalStatus = status;
          }
          if (finalStatus !== 'granted') {
            alert('Failed to get push token for push notification!');
            return;
          }
          token = (await Notifications.getExpoPushTokenAsync()).data;
        } else {
          alert('Must use physical device for Push Notifications');
        }
      
        if (Platform.OS === 'android') {
          Notifications.setNotificationChannelAsync('default', {
            name: 'default',
            importance: Notifications.AndroidImportance.MAX,
            vibrationPattern: [0, 250, 250, 250],
            lightColor: '#FF231F7C',
          });
        }

        console.log("token -> ", token);
        firebase.database().ref('users')
            .child(firebase.auth().currentUser.uid)
            .set({
                expoToken: token
            }, { merge: true });
      
        return token;
      }

    useEffect(() => {
        let subscriber;
        firebase.auth().onAuthStateChanged((user) => {
            if (user && !subscriber) {
                registerForPushNotificationsAsync().then(token => setExpoPushToken(token));
                subscriber = firebase.firestore()
                    .collection('users')
                    .doc(user.uid)
                    .onSnapshot(documentSnapshot => {
                        setUser(documentSnapshot.data())
                    })
            }
        })
        return () => subscriber();
    },[])

Im calling my function in useEffect() and im also logging the Token

console.log("token -> ", token);

Im getting two logs

One is the token is undefined and then its logs it again with the token.

The problem is, im trying to save the token to firebase:

firebase.database().ref('users')
                .child(firebase.auth().currentUser.uid)
                .set({
                    expoToken: token
                }, { merge: true });

But im getting:

[Unhandled promise rejection: Error: Reference.set failed: second argument must be a valid function.]

I think its trying to save the token but the token is undefined at the start and then it gets it.