E/AndroidRuntime: FATAL EXCEPTION: RxCachedThreadScheduler-3 Callable returned a null value

Inside my Activity B I call an async method called myASYNC that gets data from an API (the amount of data is big enough that it takes a few seconds to load all of it). If I click the Android back button that takes me back to activity A while the myASYNC is still running I get this error and the app crashes:

 E/AndroidRuntime: FATAL EXCEPTION: RxCachedThreadScheduler-3
    Process: com.hfad.crypto, PID: 9473
    java.lang.NullPointerException: Callable returned a null value. Null values are generally not allowed in 3.x operators and sources.

This happens only if I exit Activity "B" while my method is running even though it should get Disposed:

@Override
    protected void onDestroy() {
        super.onDestroy();
        if(disposableMain != null && !disposableMain.isDisposed()){
            disposableMain.dispose();
            Log.d("Disposable", "disposed");
        }
}

Activity "B"

public class ActivityB extends AppCompatActivity {
    private Disposable disposableMain;
 

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_b);
        myASYNC("DATA");
}

private void myASYNC(String data){
  Observable.fromCallable(() -> {
            try {
                URL url1 = new URL(data);
                HttpURLConnection connection = (HttpURLConnection) url1
                        .openConnection();
                connection.setDoInput(true);
                connection.connect();
                InputStream input = connection.getInputStream();
                return BitmapFactory.decodeStream(input);
            } catch (IOException e) {
                e.printStackTrace();
                Log.d("JSONTask failed", "task failed" + e.toString());
                return null;
            }
        }).subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribe(new io.reactivex.rxjava3.core.Observer<Bitmap>() {
            @Override
            public void onSubscribe(@NonNull Disposable d) {
                disposableMain = d;
            }
            @Override
            public void onNext(@NonNull Bitmap bitmap) {
                bitmapList.add(bitmap);
            }

            @Override
            public void onError(@NonNull Throwable e) {
                e.printStackTrace();
            }

            @Override
            public void onComplete() {}
        });
}


@Override
    protected void onDestroy() {
        super.onDestroy();
        if(disposableMain != null && !disposableMain.isDisposed()){
            disposableMain.dispose();
            Log.d("Disposable", "disposed");
        }
}}

How do I prevent this?