Flutter Shared Pref Returning Multiple Instances instead of one between foreground and background

Question

Everything works perfectly as long as app is running foreground.

This part is for handling background firebase message and preparing notification object and saving the received notification. This part writes json encoded notification message to shared pref

In Background or App Terminated

Future<dynamic> onBackgroundMessage(Map<String, dynamic> message) async
{
  //Custom Notification Object
  NotificationMessage test=NotificationMessage().fromMap(message);
  //Shared Pref Custom 
  PsSharedPreferences.instance.futureShared.then((value)
  {
     value.setString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE, json.encode(test.toJson()));

     PsSharedPreferences.instance.replaceNotificationMessage(json.encode(test.toJson()));

     print("Notification Message "+PsSharedPreferences.instance.getNotificationMessage());
     print("Notification Message "+value.getString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE));

  });
  //Core Shared Pref
  Future<SharedPreferences> futureShared = SharedPreferences.getInstance();
  futureShared.then((SharedPreferences shared)
  {
     NotificationMessage notificationMessage=NotificationMessage().fromMap(message);
     shared.setString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE,json.encode(notificationMessage.toJson()));

     print("Notification Message Shared ${json.encode(shared.getString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE))}");
   });
}

Logs

Notification Message Shared //Some json encoded object
Notification Message //Some json encoded object
Notification Message //Some json encoded object

In Foreground

@override
void initState()
{
   print("Notification Message "+PsSharedPreferences.instance.getNotificationMessage());
   print("Notification Message "+PsSharedPreferences.instance.shared.getString(
     Const.VALUE_HOLDER_NOTIFICATION_MESSAGE)
   );
   
   PsSharedPreferences.instance.futureShared.then((value)
   {
     //Not Working with reload as well
     value.reload();
     print("Notification Message "+value.getString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE));
     print("Notification Message "+PsSharedPreferences.instance.getNotificationMessage());
   });

   Future<SharedPreferences> futureShared = SharedPreferences.getInstance();
   futureShared.then((SharedPreferences shared)
   {
     print("Notification Message Shared 
     ${json.encode(shared.getString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE))}");
     notificationMessage=NotificationMessage.fromJson(json.decode(
        shared.getString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE))
     );
   });
   super.initState();
 }

Logs

Notification Message //Nothing
Notification Message //Nothing
Notification Message //Nothing
Notification Message //Nothing
Notification Message Shared //Nothing
[ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: FormatException: Unexpected end of input (at character 1)

Custom Shared Pref

class PsSharedPreferences
{
   PsSharedPreferences()
   {
      Utils.psPrint('init PsSharePerference $hashCode');
      futureShared = SharedPreferences.getInstance();
      futureShared.then((SharedPreferences shared)
      {
         this.shared = shared;
      });
   }
   
   Future<SharedPreferences> futureShared;
   SharedPreferences shared;

   // Singleton instance
   static final PsSharedPreferences _singleton = PsSharedPreferences();

   // Singleton accessor
   static PsSharedPreferences get instance => _singleton;

   Future<dynamic> replaceNotificationMessage(String message) async
   {
      await shared.setString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE, message);
   }

   String getNotificationMessage()
   {
      return shared.getString(Const.VALUE_HOLDER_NOTIFICATION_MESSAGE);
   }
}

Now the question is am i getting different instances of SharedPref, which is why I am getting empty result, or I have done something wrong. And In foreground everything is perfect.