public class MainActivity extends AppCompatActivity implements GridViewClickListenerSetter.PermissionCheckOnClicked{
private static final String LOG = "myLog";

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

    ViewPager viewPager = findViewById(R.id.viewPager);
    MainFragmentPagerAdapter adapter = new MainFragmentPagerAdapter(this, getSupportFragmentManager());
    viewPager.setAdapter(adapter);

    TabLayout tabLayout = findViewById(R.id.tabLayout);
    tabLayout.setupWithViewPager(viewPager);

    verifyStoragePermissions();
}

// Storage Permissions
private static final int REQUEST_EXTERNAL_STORAGE = 1;
private static String[] PERMISSIONS_STORAGE = {
        Manifest.permission.READ_EXTERNAL_STORAGE,
        Manifest.permission.WRITE_EXTERNAL_STORAGE
};

private void verifyStoragePermissions() {
    // Check if we have write permission
    int permission = ActivityCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE);

    if (permission != PackageManager.PERMISSION_GRANTED) {
        // We don't have permission so prompt the user
        ActivityCompat.requestPermissions(
                this,
                PERMISSIONS_STORAGE,
                REQUEST_EXTERNAL_STORAGE
        );
    }
}

@Override
public void itemViewClicked() {
    verifyStoragePermissions();
}

public class FragmentBeginner extends Fragment {

private BooksAdapter booksAdapter;
private boolean isCISlanguage = false;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    String lang = Locale.getDefault().getLanguage();
    if(lang.equals("ru") || lang.equals("kk") || lang.equals("kz") || lang.equals("uk")){
        booksAdapter = new BooksAdapter(getActivity(), BooksUrls.getBeginnerRu());
        isCISlanguage = true;
    }else{
        booksAdapter = new BooksAdapter(getActivity(), BooksUrls.getBeginnerEng());
        isCISlanguage = false;
    }
}

public FragmentBeginner() {
}


@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_beginner, container, false);
    GridView gridView = view.findViewById(R.id.beginnerGridView);
    gridView.setAdapter(booksAdapter);
    gridView.setOnItemClickListener(itemClickListener);
    return view;
}

GridViewClickListenerSetter itemClickListener = new GridViewClickListenerSetter(getActivity(), isCISlanguage, ((MainActivity)getActivity()));

public class GridViewClickListenerSetter implements GridView.OnItemClickListener{

private final Context context;
private boolean isCISlanguage = false;
private PermissionCheckOnClicked listener;

public GridViewClickListenerSetter(Context context, boolean isCISlanguage, PermissionCheckOnClicked listener){
    this.context = context;
    this.listener = listener;
    this.isCISlanguage = isCISlanguage;
}

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        listener.itemViewClicked();
        Book book;
        if(isCISlanguage){
            book = BooksUrls.getBeginnerRu(position);
        }else{
            book = BooksUrls.getBeginnerEng(position);
        }
        context.startService(BookDownloadService.getDownloadService(
            context,
            book.getUrl(),
            Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS) + "/"));
    }

public interface PermissionCheckOnClicked{
    void itemViewClicked();
}

java.lang.NullPointerException: Attempt to invoke interface method 'void com.studio.houmi.firebasesample.GridViewClickListenerSetter$PermissionCheckOnClicked.itemViewClicked()' on a null object reference
                                                                                 at com.studio.houmi.firebasesample.GridViewClickListenerSetter.onItemClick(GridViewClickListenerSetter.java:27)
                                                                                 at android.widget.AdapterView.performItemClick(AdapterView.java:346)
                                                                                 at android.widget.AbsListView.performItemClick(AbsListView.java:1576)
                                                                                 at android.widget.AbsListView$PerformClick.run(AbsListView.java:3962)
                                                                                 at android.widget.AbsListView$7.run(AbsListView.java:6042)
                                                                                 at android.os.Handler.handleCallback(Handler.java:739)
                                                                                 at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                 at android.os.Looper.loop(Looper.java:148)
                                                                                 at android.app.ActivityThread.main(ActivityThread.java:7329)
                                                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)