👨🏻‍💻 Ошибка при установке Progress Bar progressBar.setVisibility(View.VISIBLE)

Помогите с задачей, пожалуйста. Выдает такую ошибку

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference
    at com.example.mvvm_test.view.MainActivity$4.onChanged(MainActivity.java:94)
    at com.example.mvvm_test.view.MainActivity$4.onChanged(MainActivity.java:91)

Ругает за строчки с ProgressBar

mMyViewModel.getShowLoadingLiveData().observe(this, new Observer<Void>() {
@Override
public void onChanged(Void unused) {
progressBar.setVisibility(View.VISIBLE);
}
});
mMyViewModel.getHideLoadingLiveData().observe(this, new Observer<Void>() {
@Override
public void onChanged(Void unused) {
progressBar.setVisibility(View.GONE);
}
});



public class MyViewModel extends AndroidViewModel {
private final MutableLiveData<NumbersResponsePojo> pojoLiveData = new MutableLiveData<>();
private LiveData<List<Numbers>> numbers = new MutableLiveData<>();
private MutableLiveData <String> errorMessageLiveData = new MutableLiveData<>();
private final MutableLiveData<Void> showLoadingLiveData = new MutableLiveData<>();
private final MutableLiveData<Void> hideLoadingLiveData = new MutableLiveData<>();

private final WebRepo numberRepository;
private final DataBaseRepo dataBaseRepo;

public MyViewModel(Application application, WebRepo numberRepository) {
super(application);
dataBaseRepo = new DataBaseRepo(application);
numbers = dataBaseRepo.getAllFacts();
this.numberRepository = numberRepository;
}

void insert(Numbers numbers) {
dataBaseRepo.insert(numbers);
}

public void setPojoLiveData(NumbersResponsePojo pojoLiveData) {
setIsLoading(true);
this.pojoLiveData.setValue(pojoLiveData);
}

private void setIsLoading(boolean loading) {
if (loading) {
showLoadingLiveData.postValue(null);
} else {
hideLoadingLiveData.postValue(null);
}
}

public void getNumbers(String num) {
numberRepository.getNumbers(num)
.subscribe(new Observer<NumbersResponsePojo>() {
@Override
public void onSubscribe(Disposable d) {
}

@Override
public void onNext(NumbersResponsePojo numbersResponsePojo) {
Numbers numbers = new Numbers();
numbers.facts = numbersResponsePojo.getText();
numbers.number = numbersResponsePojo.getNumber();
numbers.time = System.currentTimeMillis();
setPojoLiveData(numbersResponsePojo);
insert(numbers);

setIsLoading(false);
//errorMessageLiveData.postValue("There is not items!");
}

@Override
public void onError(Throwable e) {
setIsLoading(false);
errorMessageLiveData.postValue("Internet is missing");
}

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

public void getRandomNumbers(String num) {
numberRepository.getRandomNumbers(num)
.subscribe(new Observer<NumbersResponsePojo>() {
@Override
public void onSubscribe(Disposable d) {
}