GetActivity () De fragmento a fragmento que devuelve nula

Tengo una vista de reciclaje, quiero que se abra un nuevo fragmento mientras hago clic en el elemento en la vista de reciclaje. getActivity () devuelve nulo cuando accedo a él en el método onCreateView, aunque cuando accedo desde el método onItemClick para pasar a otro fragmento, devuelve una excepción de puntero nulo.

public class DailyMenuFrag extends Fragment implements 

    DailyDataAdapter.onItemClickListener {

            private List<DailyData> daily_data_list;
            public List<DailyData> popular_data_list;
            DailyDataAdapter adapter = null;
            Activity activity;

            @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                     Bundle savedInstanceState) {
                daily_data_list = new ArrayList<>();
                popular_data_list = new ArrayList<>();
                load_data();
                View view = inflater.inflate(R.layout.fragment_daily_menu, container, false);
                RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.recycler_view);

                adapter = new DailyDataAdapter(getActivity(), daily_data_list);
                if(getActivity() == null){
                    Log.i("info","null");
                }
                else Log.i("info","notnull");
                adapter.setOnItemClickListner(new DailyMenuFrag());
                recyclerView.setAdapter(adapter);

                GridLayoutManager gridLayoutManager = new GridLayoutManager(getActivity(),2);
                recyclerView.setLayoutManager(gridLayoutManager);

                return view;
            }

            public void load_data() {
                task.execute("http://yemekapp.kuarkdijital.com.tr/home.php");
            }
                AsyncTask<String, Void, String> task = new AsyncTask<String, Void, String>() {

                    @Override
                    protected String doInBackground(String... params) {

                        URL url;
                        HttpURLConnection URLConnection = null;
                        String current = "";

                        try {
                            url = new URL(params[0]);

                            URLConnection = (HttpURLConnection) url.openConnection();
                            URLConnection.connect();

                            InputStream inputStream = URLConnection.getInputStream();

                            InputStreamReader reader = new InputStreamReader(inputStream);

                            int data = reader.read();

                            while (data != -1) {
                                current += (char) data;
                                data = reader.read();
                            }

                            JSONObject dailyObject = null;
                            JSONObject popularObject = null;

                            JSONObject jsonObject = new JSONObject(current);
                            JSONObject banner = jsonObject.getJSONObject("banner");

                            String daily = jsonObject.getString("daily");
                            String popular = jsonObject.getString("popular");

                            JSONArray dailyArray = new JSONArray(daily);
                            JSONArray popularArray = new JSONArray(popular);

                            for (int i = 0; i < dailyArray.length(); i++) {

                                dailyObject = dailyArray.getJSONObject(i);
                                popularObject = popularArray.getJSONObject(i);

                                DailyData DailyData = new DailyData(dailyObject.getInt("id"), dailyObject.getString("Servings"), dailyObject.getString("Title"), dailyObject.getString("CookTime"), dailyObject.getString("Image"));
                                DailyData PopularData = new DailyData(popularObject.getInt("id"), popularObject.getString("Servings"), popularObject.getString("Title"), popularObject.getString("CookTime"), popularObject.getString("Image"));
                                daily_data_list.add(DailyData);
                                popular_data_list.add(PopularData);
                                }

                            }
                            catch (JSONException e) {
                                e.printStackTrace();
                            }
                            catch (MalformedURLException e) {
                                e.printStackTrace();
                            } catch (IOException e) {
                                 e.printStackTrace();
                        }

                        return current;
                    }

                    @Override
                    protected void onPostExecute(String s) {
                        super.onPostExecute(s);
                        adapter.notifyDataSetChanged();


                    }
                };
            @Override
            public void onAttach(Activity activity) {
                super.onAttach(activity);
                this.activity = activity;
            }

            @Override
            public void onItemClick(int position) {
                Log.i("info","clicked");
                if(getActivity() == null){
                    Log.i("info","null");//why it is null here
                }
                else Log.i("info","notnull");
                DetailViewPager detailViewPager = new DetailViewPager();
        //        FragmentManager manager = getActivity().getSupportFragmentManager();
        //        FragmentTransaction transaction = manager.beginTransaction();
        //        transaction.replace(R.id.fragment_container,detailViewPager);
        //        transaction.commit();
            }
        }
   }

Error de registro

02-22 08:49:26.186 20849-20849/com.nejat.yemektarifiproject E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              Process: com.nejat.yemektarifiproject, PID: 20849
                                                                              java.lang.NullPointerException: Attempt to invoke virtual method 'android.support.v4.app.FragmentManager android.support.v4.app.FragmentActivity.getSupportFragmentManager()' on a null object reference
                                                                                  at com.nejat.yemektarifiproject.DailyMenuFrag$override.onItemClick(DailyMenuFrag.java:151)
                                                                                  at com.nejat.yemektarifiproject.DailyMenuFrag$override.access$dispatch(DailyMenuFrag.java)
                                                                                  at com.nejat.yemektarifiproject.DailyMenuFrag.onItemClick(DailyMenuFrag.java:0)
                                                                                  at com.nejat.yemektarifiproject.DailyDataAdapter$ViewHolder$1.onClick(DailyDataAdapter.java:79)
                                                                                  at android.view.View.performClick(View.java:5637)