Flutter: FCM send notification to multiple devices not working

I have connected my project to firebase for FCM notification, and build this function to send notification for all users of my app if the admin press a special button:

void sendNotificationToAllUsers() async {
    var url = Uri.parse('https://fcm.googleapis.com/fcm/send');
    const serverKey = 'XXXXXXXXXXXXXXXXXXXX';

    var headers = {
      'Content-Type': 'application/json',
      'Authorization': 'key=$serverKey',
    };

    var body = {
      'notification': {
        'title': 'Hey guys',
        'body': 'How are you?!',
      },
      'to': '/topics/all',
      'click_action' : 'FLUTTER_NOTIFICATION_CLICK'
    };

    try {
      var response = await http.post(
        url,
        headers: headers,
        body: jsonEncode(body),
      );
      if (response.statusCode == 200) {
        print('Notification sent successfully');
      } else {
        print('Failed to send notification: ${response.reasonPhrase}');
      }
    } catch (e) {
      print('Failed to send notification: $e');
    }
  }

and I have added this script to the AndroidManafist.xml:

            <intent-filter>
                <action android:name="FLUTTER_NOTIFICATION_CLICK"/>
                <category android:name="android.intent.category.DEFAULT"/>
            </intent-filter>

but it prints this in the console: Notification sent successfully without sending any notifications!!!

I really searched for that, but didn't catch ideas about sending for many devices in one push from the app not from Firebase console!!!

any help would be appreciated.

Update:

I have been added this line in the very first of the "sendNotificationToAllUsers" function:

await FirebaseMessaging.instance.subscribeToTopic('all');

But it still prints Notification sent successfully without sending the notification !!!

I had solved that by updating that function to this:

void sendNotificationToAllUsers() async {
    final messaging = FirebaseMessaging.instance;
    await messaging.subscribeToTopic('all');
    await messaging.requestPermission();

    var url = Uri.parse('https://fcm.googleapis.com/fcm/send');
    const serverKey = 'XXXXXXXXXXXXXXXXXXXX';

    var headers = {
      'Content-Type': 'application/json',
      'Authorization': 'key=$serverKey',
    };

    var data = {
      'notification': {
        'title': notifyTitle.text,
        'body': notifyText.text,
      },
      'priority': 'high',
      'to': "/topics/all",
      'click_action': 'FLUTTER_NOTIFICATION_CLICK'
    };

    try {
      var response = await http.post(
        url,
        headers: headers,
        body: jsonEncode(data),
      );
      if (response.statusCode == 200) {
        debugPrint('Notification sent successfully');
      } else {
        debugPrint('Failed to send notification: ${response.reasonPhrase}');
      }
    } catch (e) {
      debugPrint('Failed to send notification: $e');
    }
    clearControllers();
    update();
  }