Failed to post notification on channel "null" Target Api is 26

Gulzar Bhat's answer is working perfectly if your minimum API is Oreo. If your minimum is lower however, you have to wrap the NotificationChannel code in a platform level check. After that you can still use the id, which will be ignored pre Oreo:

if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
    int importance = NotificationManager.IMPORTANCE_LOW;
    NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, NOTIFICATION_CHANNEL_NAME, importance);
    notificationChannel.enableLights(true);
    notificationChannel.setLightColor(Color.RED);
    notificationChannel.enableVibration(true);
    notificationChannel.setVibrationPattern(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});
    notificationManager.createNotificationChannel(notificationChannel);
}

NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context, NOTIFICATION_CHANNEL_ID);

NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
notificationManager.notify((int)(System.currentTimeMillis()/1000), mBuilder.build());

First create the notification channel:

public static final String NOTIFICATION_CHANNEL_ID = "4655";
//Notification Channel
        CharSequence channelName = NOTIFICATION_CHANNEL_NAME;
        int importance = NotificationManager.IMPORTANCE_LOW;
        NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, NOTIFICATION_CHANNEL_NAME, importance);
        notificationChannel.enableLights(true);
        notificationChannel.setLightColor(Color.RED);
        notificationChannel.enableVibration(true);
        notificationChannel.setVibrationPattern(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});


NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannel(notificationChannel);

then use the channel id in the constructor:

final NotificationCompat.Builder builder = new NotificationCompat.Builder(context, NOTIFICATION_CHANNEL_ID)
                .setDefaults(Notification.DEFAULT_ALL)
                .setSmallIcon(R.drawable.ic_timers)
                .setVibrate(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400})
                .setSound(null)
                .setContent(contentView)
                .setPriority(NotificationCompat.PRIORITY_DEFAULT)
                .setLargeIcon(picture)
                .setTicker(sTimer)
                .setContentIntent(timerListIntent)
                .setAutoCancel(false);

This issue is related to the old FCM version.

Update your dependency to com.google.firebase:firebase-messaging:15.0.2 or higher.

This will fix the error

failed to post notification on channel null

when your app receives notifications in the background because now Firebase provides a default notification channel with basic settings.

But you also can specify the default Notification Channel for FCM in the manifest.

<meta-data
android:name="com.google.firebase.messaging.default_notification_channel_id"
android:value="@string/default_notification_channel_id"/>

To learn more here

If you get this error should be paid attention to 2 items and them order:

1. NotificationChannel mChannel = new NotificationChannel(id, name, importance);
2. builder = new NotificationCompat.Builder(this, id);

Also NotificationManager notifManager and NotificationChannel mChannel are created only once.

There are required setters for Notification:

builder.setContentTitle() // required  
       .setSmallIcon()    // required 
       .setContentText()  // required  

See the example in On Android 8.1 API 27 notification does not display.

I had a similar problem but starting an service as a background activity, in the service this code worked:

public static final int PRIMARY_FOREGROUND_NOTIF_SERVICE_ID = 1001;

@Override
public void onCreate() {
    super.onCreate();

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {

        String id = "_channel_01";
        int importance = NotificationManager.IMPORTANCE_LOW;
        NotificationChannel mChannel = new NotificationChannel(id, "notification", importance);
        mChannel.enableLights(true);

        Notification notification = new Notification.Builder(getApplicationContext(), id)
                .setSmallIcon(R.drawable.ic_launcher)
                .setContentTitle("My chat")
                .setContentText("Listening for incoming messages")
                .build();

        NotificationManager mNotificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        if (mNotificationManager != null) {
            mNotificationManager.createNotificationChannel(mChannel);
            mNotificationManager.notify(PRIMARY_FOREGROUND_NOTIF_SERVICE_ID, notification);
        }

        startForeground(PRIMARY_FOREGROUND_NOTIF_SERVICE_ID, notification);
    }
}

It's not the best solution but just to start the service in the background It does work

String CHANNEL_ID = "my_channel";

NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this,CHANNEL_ID);