Android O Foreground Service - How to handle the communcation with activity even when app is kiiled from background or get restarted

Question

I have a foreground service in which it needs to check whether my activity is running or not. If running then update its UI otherwise it should not crash. Scenario goes like whenever user login, I am starting a foreground service which sync the data for users and keep updating the UI of multiple activity accordingly. I have used broadcast receiver to establish a communication between service and activity but as soon the app getting killed by user and reopen i m getting below Exception

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.content.BroadcastReceiver.onReceive(android.content.Context, android.content.Intent)' on a null object reference
    at android.support.v4.content.LocalBroadcastManager.executePendingBroadcasts(LocalBroadcastManager.java:313)
    at android.support.v4.content.LocalBroadcastManager$1.handleMessage(LocalBroadcastManager.java:121)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6863)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:537)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Below is sample of MyActivity code :

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.support.v4.content.LocalBroadcastManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import sample.androido.com.myapplication.R;

public class CreateForegroundServiceActivity extends AppCompatActivity {
    TextView display;
    BroadcastReceiver broadcastReceiver= new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            switch (intent.getAction()) {
                case "CONNECTED":
                    display.setText("Service connected");
                    break;
            }

        }
    };;
    private boolean isReceiverRegistered = false;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_create_foreground_service);
        Button startServiceButton = (Button)findViewById(R.id.start_foreground_service_button);
        startServiceButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(CreateForegroundServiceActivity.this, MyForeGroundService.class);
                intent.setAction(MyForeGroundService.ACTION_START_FOREGROUND_SERVICE);
                startService(intent);
            }
        });

        Button stopServiceButton = (Button)findViewById(R.id.stop_foreground_service_button);
        stopServiceButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(CreateForegroundServiceActivity.this, MyForeGroundService.class);
                intent.setAction(MyForeGroundService.ACTION_STOP_FOREGROUND_SERVICE);
                startService(intent);
            }
        });

             display  = findViewById(R.id.display);

    }


    private void registerReceiver() {

        if(!isReceiverRegistered){
            IntentFilter intentFilter = new IntentFilter();
            intentFilter.addAction("CONNECTED");
            LocalBroadcastManager.getInstance(this).registerReceiver(broadcastReceiver,intentFilter);
            isReceiverRegistered = true;
        }
    }
    private void unregisterReceiver() {
        if(broadcastReceiver != null) {
            LocalBroadcastManager.getInstance(this).unregisterReceiver(broadcastReceiver);
            broadcastReceiver = null;
            isReceiverRegistered = false;

        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        registerReceiver();

    }
    @Override
    public void onPause() {
        super.onPause();
        unregisterReceiver();

    }
}

Below is my Service code sample :

import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.ContentResolver;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.media.AudioAttributes;
import android.net.Uri;
import android.os.Build;
import android.os.Handler;
import android.os.IBinder;
import android.support.v4.content.LocalBroadcastManager;
import android.util.Log;
import android.widget.Toast;
import android.support.v4.app.NotificationCompat;

import sample.androido.com.myapplication.R;

public class MyForeGroundService  extends Service {
    private static final String TAG_FOREGROUND_SERVICE = "FOREGROUND_SERVICE";

    public static final String ACTION_START_FOREGROUND_SERVICE = "ACTION_START_FOREGROUND_SERVICE";

    public static final String ACTION_STOP_FOREGROUND_SERVICE = "ACTION_STOP_FOREGROUND_SERVICE";

    public static final String ACTION_PAUSE = "ACTION_PAUSE";

    public static final String ACTION_PLAY = "ACTION_PLAY";
    public static final String PRIMARY_CHANNEL = "default";

    public MyForeGroundService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
    }

    @Override
    public void onCreate() {
        super.onCreate();
        Log.d(TAG_FOREGROUND_SERVICE, "My foreground service onCreate().");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if(intent != null)
        {
            String action = intent.getAction();

            switch (action)
            {
                case ACTION_START_FOREGROUND_SERVICE:
                    startForegroundService();
                    Toast.makeText(getApplicationContext(), "Foreground service is started.", Toast.LENGTH_LONG).show();
                    break;
                case ACTION_STOP_FOREGROUND_SERVICE:
                    stopForegroundService();
                    Toast.makeText(getApplicationContext(), "Foreground service is stopped.", Toast.LENGTH_LONG).show();
                    break;

            }
        }
        return super.onStartCommand(intent, flags, startId);
    }

    /* Used to build and start foreground service. */
    private void startForegroundService()
    {

        NotificationCompat.Builder mBuilder = notificationBuilder();
        // Start foreground service.
        startForeground(1, mBuilder.build());


        Handler handler = new Handler();
        handler.postDelayed(new Runnable() {
            @Override
            public void run() {
                Intent intent = new Intent();
                intent.setAction("CONNECTED");
                LocalBroadcastManager.getInstance(getApplicationContext()).sendBroadcast(intent);

            }
        },100000);
    }

    private void stopForegroundService()
    {
        Log.d(TAG_FOREGROUND_SERVICE, "Stop foreground service.");

        // Stop foreground service and remove the notification.
        stopForeground(true);

        // Stop the foreground service.
        stopSelf();
    }



    public NotificationCompat.Builder  notificationBuilder(){
        NotificationCompat.Builder mBuilder=  new NotificationCompat.Builder(this, PRIMARY_CHANNEL)
                .setContentTitle("Dummy Title")
                .setContentText("Dummy Message")
                .setStyle(new NotificationCompat.BigTextStyle()
                        .bigText("Big text Message"
                        ))
                .setPriority(NotificationCompat.PRIORITY_DEFAULT);

        if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            mBuilder.setSmallIcon(R.drawable.app_icon_white);
            mBuilder.setColor(getResources().getColor(R.color.theme_color));
        } else {
            mBuilder.setSmallIcon(R.drawable.app_icon_white);
        }

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            CharSequence name = getResources().getString(R.string.feroz_channel_name);
            String description = getResources().getString(R.string.feroz_channel_description);
            int importance = NotificationManager.IMPORTANCE_DEFAULT;
            NotificationChannel channel = new NotificationChannel(PRIMARY_CHANNEL, name, importance);
            channel.enableLights(true);
            channel.setLightColor(getResources().getColor(R.color.theme_color));
            channel.enableVibration(true);
            channel.setDescription(description);

            NotificationManager notificationManager1 = getSystemService(NotificationManager.class);
            notificationManager1.createNotificationChannel(channel);
        }


        Intent stopIntent = new Intent(this, MyForeGroundService.class);
        stopIntent.setAction(ACTION_STOP_FOREGROUND_SERVICE);
        PendingIntent stopPlayIntent = PendingIntent.getService(this, 0, stopIntent, 0);
        mBuilder.addAction(R.drawable.ic_launcher_new,"Hang Up",stopPlayIntent);
        Intent intent = new Intent(this, CreateForegroundServiceActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent,  PendingIntent.FLAG_UPDATE_CURRENT);
        mBuilder.setContentIntent(pendingIntent);
         mBuilder.setUsesChronometer(true);

        return mBuilder;
    }
}

I m not sure why its getting crashed. is it because of app is in Background not in foreground. Or This is not right way of communication between service and activity.