android.view.View android.view.View.findViewById(int)' on a null How to fix this error

I don’t know how to fix this error.Repeat findViewById as a null pointer because java.lang.NullPointerException: Attempt to invoke virtual method ‘android.view.View android.view.View.findViewById(int)’ on a null object reference I have given the image clearly below, see it and fix the error i am new devloper

  public class FragmentCategory extends Fragment {
    private Call<CallbackHome> callbackCall = null;
    SharedPref sharedPref;
    private View root_view;
    private GalleryAdapter adapterCategory;
    private GridView gridView;
    private List<Category> mensWears;
    private GalleryAdapter adapter;

    public FragmentCategory() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_phones,container,false);
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view,savedInstanceState);

        this.callbackCall = RestAdapter.createAPI(getApiUrl).getHome(AppConfig.REST_API_KEY);
        this.callbackCall.enqueue(new Callback<CallbackHome>() {
            public void onResponse(Call<CallbackHome> call, Response<CallbackHome> response) {
                CallbackHome responseHome = response.body();
                if (responseHome == null || !responseHome.status.equals("ok")) {

                    return;
                }
                displayData(responseHome);


            }

            private void displayData(CallbackHome responseHome) {

                displayCategory(responseHome.category);


            }


            public void onFailure(Call<CallbackHome> call, Throwable th) {
                Log.e("onFailure", th.getMessage());
                if (!call.isCanceled()) {

                }
            }
        });



    }

    private void displayCategory(List<Category> list) {
        GridView gridView = (GridView) root_view.findViewById(R.id.gridHolder);
        gridView.setAdapter(adapterCategory);

       
        
        adapterCategory = new GalleryAdapter(getActivity(), new ArrayList<>());
        gridView.setAdapter(adapterCategory);

        LinearLayout lyt_category = root_view.findViewById(R.id.lyt_category);
        if (list.size() > 0) {
            lyt_category.setVisibility(View.VISIBLE);
        } else {
            lyt_category.setVisibility(View.GONE);
        }
    }

>Solution :

Replace your onCreateView code with this

@Override
public View onCreateView(LayoutInflater inflater,ViewGroup container,
                     Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    root_view = inflater.inflate(R.layout.fragment_phones,container,false);
    return root_view
}

You forgot to initialize your instance variable private View root_view;