Cannot create an instance of class ViewModel

I am trying to write a sample app using Android architecture components and but even after trying for days I could not get it to work. It gives me the above exception.

Lifecycle owner:-

public class MainActivity extends LifecycleActivity {

    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = findViewById(R.id.tv_user);
        PostViewModel viewModel = ViewModelProviders.of(this).get(PostViewModel.class);
        viewModel.loadPosts();
        viewModel.getPost().observe(this, new Observer<Post>() {
            @Override
            public void onChanged(@Nullable Post post) {
                if(post != null) {
                    textView.setText(post.toString());
                }
            }
        });
    }
}

ViewModel:-

public class PostViewModel extends ViewModel {
    private MediatorLiveData<Post> post;
    private PostRepository postRepo;

    PostViewModel() {
        post = new MediatorLiveData<>();
        postRepo = new PostRepository();
    }

    public LiveData<Post> loadPosts() {
        post.addSource(postRepo.getPost(),
                post -> this.post.setValue(post)
        );
        return post;
    }

    @NonNull
    public LiveData<Post> getPost() {
        return post;
    }
}

Make your constructor public.