NullPointerException occured when trying to subscribe to nats.io topic

Question

I'm evaluating whether nats.io is an okay replacement for Firebase Cloud Messaging or not.

This is my started code which uses nats-android library

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;

import com.google.android.material.button.MaterialButton;

import java.io.IOException;
import java.nio.charset.StandardCharsets;

import io.nats.client.Connection;
import io.nats.client.Duration;
import io.nats.client.Message;
import io.nats.client.Nats;
import io.nats.client.Options;
import io.nats.client.Subscription;

public class MainActivity extends AppCompatActivity {

    private Connection nc;
    private MaterialButton btnPublish, btnSubscribe;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btnPublish = (MaterialButton)  findViewById(R.id.btn_nats_publish);
        btnSubscribe = (MaterialButton) findViewById(R.id.btn_nats_subscribe);

        Options.Builder oBuilder =
                new Options.Builder()
                        .server(Constants.NATS_BASE_URL)
                        .reconnectWait(Duration.ofMinutes(3)).maxReconnects(-1);
        Options opts = oBuilder.build();

        try {
            nc = Nats.connect(opts);
        }
        catch (InterruptedException ie){
            Log.d("NATS", "InterruptedException at MainActivity: "+ie.getMessage());
        }
        catch (IOException ioe){
            Log.d("NATS", "IOException at MainActivity: "+ ioe.getMessage());
        }
        if (!NotificationService.natsSvcIsRunning){
            Intent start = new Intent(MainActivity.this.getApplicationContext(), NotificationService.class);
            start.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            start.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
            MainActivity.this.startService(start);
        }

        btnPublish.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Log.d("NATS", "Trying to publish...");
                nc.publish("rcpt", "msg".getBytes(StandardCharsets.UTF_8));
            }
        });

        btnSubscribe.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Subscription sub = nc.subscribe("topic");
                Log.d("NATS","subscribing to "+sub.getSubject());
                Message msg = null;

                try {
                    msg = sub.nextMessage(Duration.ZERO);
                    String response = new String(msg.getData(), StandardCharsets.UTF_8);
                    Log.d("NATS","got request "+response+" from "+msg.getReplyTo());
                }
                catch (InterruptedException ie){
                    Log.d("NATS", "InterruptedException: "+ie.getMessage());
                }
            }
        });
    }
}

Running the code, here's the logcat output

2022-01-11 15:54:17.689 16748-16748/com.anta40.app.natsdemo D/NATS: IOException at MainActivity: Unable to connect to NATS server.
2022-01-11 15:54:17.711 16748-16748/com.anta40.app.natsdemo D/NATS: NotificationService onCreate() method.
..
..
..
2022-01-11 15:54:17.800 16748-16787/com.anta40.app.natsdemo D/NATS: nats server connection nats: connection opened
2022-01-11 15:54:17.801 16748-16774/com.anta40.app.natsdemo D/NATS: subscribing to req_auth

I'm aware that:

1. There's an IOException occured in MainActivity
2. NATS subscription is succesfully done in NotificationService

Not surprisingly, when you click the subscribe button, a NullPointerException will be triggered.

My question is how to create NATS connection in MainActivity properly?