Failed to post notification on channel “null”

Since Android Oreo, you need a Channel to send your notifications. You can do something like this (not tested) :

public class FirebaseMessagingService extends com.google.firebase.messaging.FirebaseMessagingService {

  public static final String NOTIFICATION_CHANNEL_ID = "10001";

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        super.onMessageReceived(remoteMessage);

        String notification_title = remoteMessage.getNotification().getTitle();
        String notification_message = remoteMessage.getNotification().getBody();
        String click_action = remoteMessage.getNotification().getClickAction();
        String from_user_id = remoteMessage.getData().get("from_user_id");

        NotificationCompat.Builder mBuilder =
                new NotificationCompat.Builder(this)
                        .setSmallIcon(R.mipmap.ic_launcher)
                        .setContentTitle(notification_title)
                        .setContentText(notification_message);


        Intent resultIntent = new Intent(click_action);
        resultIntent.putExtra("user_id", from_user_id);

        PendingIntent resultPendingIntent =
                PendingIntent.getActivity(
                        this,
                        0,
                        resultIntent,
                        PendingIntent.FLAG_UPDATE_CURRENT
                );

        mBuilder.setContentIntent(resultPendingIntent);
        int mNotificationId = (int) System.currentTimeMillis();
        NotificationManager mNotifyMgr =
                (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O)
        {
            int importance = NotificationManager.IMPORTANCE_HIGH;
            NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "NOTIFICATION_CHANNEL_NAME", importance);

            mBuilder.setChannelId(NOTIFICATION_CHANNEL_ID);
            mNotifyMgr.createNotificationChannel(notificationChannel);
        }
        mNotifyMgr.notify(mNotificationId, mBuilder.build());
    }
}