如何从android中的firebase中检索值？

Question

我是使用Firebase的新手。我正在尝试从Firebase获取数据，但是当我从Firebase获取数据时，我的应用程序正在崩溃。我试图从msg获取价值，我的数据结构是这样的： -

users
 -KUWbdwMC7Nw9EIqKfHH
 msg: "none"

通知服务类

public class NotificationListener extends Service {


    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }


    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Toast.makeText(NotificationListener.this, "Service Started", Toast.LENGTH_SHORT).show();


        SharedPreferences sharedPreferences = getSharedPreferences(Constants.SHARED_PREF, MODE_PRIVATE);


        String id = sharedPreferences.getString(Constants.UNIQUE_ID, null);


        Firebase firebase = new Firebase(Constants.FIREBASE_APP + id);


        firebase.addValueEventListener(new ValueEventListener() {




            @Override
            public void onDataChange(DataSnapshot snapshot) {

                String msg = snapshot.child("msg").getValue().toString();


                if (msg.equals("none"))
                    return;

                showNotification(msg);
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {
                Log.e("The read failed: ", firebaseError.getMessage());
            }
        });

        return START_STICKY;
    }


    private void showNotification(String msg){
        //Creating a notification
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this);
        builder.setSmallIcon(R.mipmap.ic_launcher);
        Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("https://www.facebook.com"));
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);
        builder.setContentIntent(pendingIntent);
        builder.setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher));
        builder.setContentTitle("Firebase Push Notification");
        builder.setContentText(msg);
        NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        notificationManager.notify(1, builder.build());
    }
}

日志：

10-20 15:59:24.525 8000-8000/com.example.pitech09.mbz E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.pitech09.mbz, PID: 8000
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
    at com.example.pitech09.mbz.NotificationListener$1.onDataChange(NotificationListener.java:55)
    at com.firebase.client.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:56)
    at com.firebase.client.core.view.DataEvent.fire(DataEvent.java:45)
    at com.firebase.client.core.view.EventRaiser$1.run(EventRaiser.java:38)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5343)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)