Notification wird nicht angezeigt

NotificationManager notificationManager = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
                Intent intentMainActivity = new Intent(this.getApplicationContext(), MainActivity.class);
                PendingIntent pendingIntentMainActivity = PendingIntent.getActivity(this, 0, intentMainActivity, 0);
                Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
                Notification popup = new Notification.Builder(this)
                        .setContentTitle("Wecker!")
                        .setContentText(uhrzeit)
                        .setContentIntent(pendingIntentMainActivity)
                        .setSmallIcon(R.drawable.alarm)
                        .setSound(defaultSoundUri)
                        .setVibrate(new long[] { 100, 250 })
                        .setAutoCancel(true)
                        .build();
                //notificationManager.notify(0, popup);
                notificationManager.notify("Wecker um " + uhrzeit, 0, popup);