Initialize loader

onCreate.java

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

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://api.github.com")
            .addConverterFactory(GsonConverterFactory.create())
            .build();

    mGithubService = retrofit.create(GithubService.class);
    mAdapter = new UsersAdapter();

    RecyclerView recyclerView = findViewById(R.id.recycler_view_users);
    recyclerView.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));
    recyclerView.setAdapter(mAdapter);

    getSupportLoaderManager().initLoader(LOADER_ID, null, this);
}