Fragment not attached to a context after asynctask

Hello everyone I have a problem in fragment, I get a null pointer exception in fragment after changing from second to third fragment:On first onCreateView it's working fine, but when i change it, i get null pointer exception. I tryed with getContext(),getActivity(),requireContext() but I always get this exception. Thanks java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference

 public class GetListaVozila extends AsyncTask<ArrayList<String>, Void, Void> {



        protected void onPreExecute() {
            /****** NOTE: You can call UI Element here. *****/
//            progressDialog = getDialogProgressBar().show();
//            progressDialog.setCancelable(false);

        }


        // Call after onPreExecute method
        protected Void doInBackground(ArrayList<String>... urls) {
            try {
                System.out.println("pozvanooo1");
                URL url = new URL("http://example/app/lista_vozila_novo.php");
                URLConnection conn = url.openConnection();
                HttpURLConnection httpConn = (HttpURLConnection) conn;
                httpConn.setAllowUserInteraction(false);
                httpConn.setInstanceFollowRedirects(true);
                httpConn.setRequestMethod("POST");
                httpConn.setDoInput(true);
                httpConn.setDoOutput(true);
                Uri.Builder builder = new Uri.Builder()
                        .appendQueryParameter("key", key);
                String query = builder.build().getEncodedQuery();
                httpConn.connect();
                OutputStream os = httpConn.getOutputStream();
                BufferedWriter writer = new BufferedWriter(
                        new OutputStreamWriter(os, "UTF-8"));
                writer.write(query);
                writer.flush();
                writer.close();
                os.close();
                InputStream is = httpConn.getInputStream();
                parsedString = convertinputStreamToString(is);
                System.out.println("pozvanooo parsedstring" + parsedString + " - " + key);


            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }



        protected void onPostExecute(Void unused) {

            Gson gson = new Gson();
            Type type = new TypeToken<List<PozicijaVozila>>() {
            }.getType();
            listaVozila = gson.fromJson(parsedString, type);
            values = new ArrayList<String>();
//            ArrayList<PozicijaVozila> array_sort = new ArrayList<PozicijaVozila>(listaVozila);
            for (PozicijaVozila p : listaVozila) {
                values.add(p.getIme());
            }
            it = new ArrayList<PozicijaVozila>(listaVozila);
//            for (int i = 0; i < it.size(); i++) {
//                values.add(it.get(i).getIme());
//            }
I got exception here: adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_activated_1, android.R.id.text1, values);
            editTextNaziv.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    final Dialog dialog = new Dialog(requireContext());
                    dialog.setContentView(R.layout.vozila_listview);
                    dialog.setTitle(requireContext().getResources().getString(R.string.izaberite_vozilo));
                    listView = (ListView) dialog.findViewById(R.id.list);
                    dialog.show();
                    adapter.notifyDataSetChanged();
                    listView.setAdapter(adapter);
                    listView.setChoiceMode(ListView.CHOICE_MODE_SINGLE);
                    listView.setItemChecked(selectedIndex, false);
                    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

                        @Override
                        public void onItemClick(AdapterView<?> parent, View view,
                                                int position, long id) {
                            int itemPosition = position;
                            selectedIndex = position;
                            String itemValue = (String) listView
                                    .getItemAtPosition(position);
                            currentSelectedView = view;
                            editTextNaziv.setText(itemValue);
                            // Show Alert
                            view.setBackgroundColor(Color.parseColor("#F8C304"));
                            dialog.cancel();

                        }

                    });


                }
            });
//            progressDialog.dismiss();
//            if (selectedIndex == 0 && values.size() > 0) {
//                editTextNaziv.setText(values.get(0));
//            }
        }
    }

looks like getActivity() is null when AsyncTask finishes its job. it is placed inside Fragment, so this means that this Fragment wasn't attached to any Activity at that particular moment (job end). you should cancel your AsyncTask in some of lifecycle methods like onPause or onDestroyView for avoiding this problem. but be awared that AsyncTask won't have a place to print result of its job (as Fragment - GUI side - was removed from screen). if you want to keep that work for some other Fragment or just for storing for future use - start and keep AsyncTask inside Activity (as you see it is tightened to it anyway by getActivity() method, not to Fragment)