How to save push notifications from fcm firebase to hive db, in the background with flutter

I have been making a flutter application and I have a notifications section, and I have been trying to save the push notifications in the Hive database in the background or when the cell phone is blocked, I don't want to use StopWatch or similar packages to this, as I think it is annoying for the user, the error I get is this:

/FLTFireMsgReceiver(31121): broadcast received for message
W/FirebaseMessaging(31121): Missing Default Notification Channel metadata in AndroidManifest. Default value will be used.
I/flutter (31121): 
E/flutter (31121): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: HiveError: You need to initialize Hive or provide a path to store the box.
E/flutter (31121): #0      BackendManager.open (package:hive/src/backend/vm/backend_manager.dart:16:7)
E/flutter (31121): #1      HiveImpl._openBox (package:hive/src/hive_impl.dart:106:30)
E/flutter (31121): #2      HiveImpl.openBox (package:hive/src/hive_impl.dart:145:18)
E/flutter (31121): #3      DbProviderHive.openBoxNotification (package:myproyectapp/src/services/database/hive/db_provider_hive.dart:80:42)
E/flutter (31121): <asynchronous suspension>
E/flutter (31121): #4      NotificationDao._getDbInstance (package:myproyectapp/src/services/database/daos/nofication_dao.dart:14:11)
E/flutter (31121): <asynchronous suspension>
E/flutter (31121): #5      NotificationDao.insert.<anonymous closure> (package:myproyectapp/src/services/database/daos/nofication_dao.dart:20:9)
E/flutter (31121): <asynchronous suspension>
E/flutter (31121): #6      _QueuedFuture.execute (package:queue/src/dart_queue_base.dart:26:16)
E/flutter (31121): <asynchronous suspension>
E/flutter (31121): 
I/flutter (31121): ----------------FIREBASE CRASHLYTICS----------------
I/flutter (31121): The following exception was thrown Internal Record Error:
I/flutter (31121): MissingPluginException(No implementation found for method getAll on channel plugins.flutter.io/shared_preferences)
I/flutter (31121): 
I/flutter (31121): ----------------------------------------------------

And this is my code where I get the DB instance, when the app is in foreground it works correctly for me.

class DbProviderHive {
  static final DbProviderHive _singleton = DbProviderHive._internal();

  static Box<NotificationModel> _boxNotificationModel;

  factory DbProviderHive() {
    return _singleton;
  }

  DbProviderHive._internal();

  static Future<void> _initSettings() async {
    // Se encarga de guardar la BD en un directorio valido
    final folderName = 'myDB';

    final databasePath = await getDatabasesPath();
    if (await Directory(join(databasePath, folderName)).exists()) {
      await Directory(join(databasePath, folderName)).delete(recursive: true);
    }

    await Hive.initFlutter(join(databasePath, folderName));
  }

static Future<void> _initHiveDatabase() async {
    try {
      await _initSettings();
      Hive.registerAdapter(NotificationModelAdapter()); // TypeId: 0
      print("");
    } catch (e) {
      debugPrint("");
    }

  static Future<Box<NotificationModel>> openBoxNotification() async {
    if (_boxNotificationModel == null) {
      await _initHiveDatabase();
      _boxNotificationModel = await Hive.openBox<NotificationModel>(NotificationModel.tableName);
    }

    if (!_boxNotificationModel.isOpen) {
      _boxNotificationModel = await Hive.openBox<NotificationModel>(NotificationModel.tableName);
    }
    return _boxNotificationModel;
  }

  static Future<void> dispose() async {
    await Hive.close();
  }
}

Do you have any idea how to save the notifications in the background?