I am getting this error-Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
This is what i have done!
PeopleListFragment
 **PeopleListFragment** 
public class PeopleListFragment extends Fragment{
    public static final String DISPLAY_URL = ServiceAPI.URL_CONTACTS;
    private static final String TAG = "PeopleListActivity";
    public static final boolean DEBUG = Constant.DEBUG;
    private ProgressDialog progressDialog;
    public PeopleAdapter  adapter;
    private GetAllPeopleTask mGetAllPeople = null;
    private Intent intent;
    public static ListView mContactlistView;
    public PeopleListFragment() {
        // 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_people_list, container, false);
    }
    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        mContactlistView = (ListView) view.findViewById(R.id.people_list);
    }
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        List<People> contactList = new ArrayList<People>();
        adapter = new PeopleAdapter(getActivity().getApplicationContext(),
                R.layout.people_list_item, contactList);
        mContactlistView.setAdapter(adapter);
        mContactlistView.setTextFilterEnabled(true);
        mContactlistView.setFastScrollEnabled(true);
        mContactlistView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String empId = String.valueOf(adapter.getItem(position).getEmployeeId());
                String name = String.valueOf(adapter.getItem(position).getFirstName());
                intent = new Intent(getActivity(),
                        PeopleDetailsActivity.class);
                intent.putExtra("EmployeeId", empId);
                intent.putExtra("name", name);
                startActivity(intent);
            }
        });
        startAsyncTask();
        TextView textView = new TextView(getContext());
        textView.setText("Empty");
        mContactlistView.setEmptyView(textView);
    }
    public void startAsyncTask() {
        mGetAllPeople = new GetAllPeopleTask();
        mGetAllPeople.execute(DISPLAY_URL, "12345678");
    }
    public class GetAllPeopleTask extends AsyncTask<String, Void, List<People>> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressDialog = new ProgressDialog(getActivity());
            progressDialog.setMessage("Loading");
            progressDialog.show();
        }
        @Override
        protected ArrayList<People> doInBackground(String... params) {
            ArrayList<People> peopleList = new ArrayList<>();
            try {
                URL url = new URL(params[0]);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.setReadTimeout(40000);
                httpURLConnection.setConnectTimeout(50000);
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setRequestProperty("Content-Type", "application/json");
                httpURLConnection.setRequestProperty("Accept", "application/json");
                httpURLConnection.setDoOutput(true);
                JSONObject request = new JSONObject();
                request.put("session", params[1]);
                String bodyParams = request.toString();
                if (DEBUG) {
                    Log.i(TAG, bodyParams);
                }
                httpURLConnection.setRequestProperty("Content-Length",
                        String.valueOf(bodyParams.getBytes().length));
                DataOutputStream outputStream = new DataOutputStream(httpURLConnection.getOutputStream());
                outputStream.writeBytes(bodyParams);
                outputStream.flush();
                outputStream.close();
                httpURLConnection.connect();
                if (httpURLConnection.getResponseCode() == 200) {
                    Log.i(TAG, "Success");
                } else {
                    Log.i(TAG, "Error  Code :" + httpURLConnection.getResponseCode());
                }
                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
                String line;
                StringBuilder builder = new StringBuilder();
                while ((line = reader.readLine()) != null) {
                    builder.append(line);
                }
                Log.i(TAG, "Builder String : " + builder.toString());
                JSONObject jsonObject = new JSONObject(builder.toString());
                Log.i(TAG, "Response JSON : " + jsonObject.toString());
                if (jsonObject.has("responseData")) {
                    JSONArray responseDataArr = jsonObject.optJSONArray("responseData");
                    for (int i = 0; i < responseDataArr.length(); i++) {
                        JSONObject peopleJSON = responseDataArr.optJSONObject(i);
                        People people = new People();
                        people.setEmployeeId(peopleJSON.optString("employeeId"));
                        people.setFirstName(peopleJSON.optString("firstName"));
                        people.setLastName(peopleJSON.optString("lastName"));
                       //  Log.i("hebsiba ", "People first name" + people.getFirstName() + ", last name is " + people.getLastName()) ;
                        if((!people.getFirstName().isEmpty()) && (!people.getLastName().isEmpty())) {
                            peopleList.add(people);
                        }
                        if (DEBUG) {
                            Log.i(TAG, "People " + people.getEmployeeId());
                        }
                    }
                }
            } catch (IOException | JSONException e) {
                Log.e(TAG, e.toString());
            }
            return peopleList;
        }
        @Override
        protected void onPostExecute(List<People> people) {
            super.onPostExecute(people);
            Collections.sort(people, new Comparator<People>() {
                @Override
                public int compare(People lhs, People rhs) {
                    return lhs.getFirstName().compareToIgnoreCase(rhs.getFirstName());
                }
            });
            adapter.addAll(people);
            if (DEBUG) {
                Log.i(TAG, adapter.getCount() + " ");
            }
            Log.v(TAG, String.valueOf(people.size()));
            progressDialog.dismiss();
        }
    }
}
This is the PeopleAdapter
 **PeopleAdapter** 
public class PeopleAdapter extends ArrayAdapter<People> implements Filterable {
    private PeopleListActivity activity;
    private PeopleFilter peopleFilter;
    private List<People> peopleList;
    private List<People> filteredList;
    public PeopleAdapter(Context applicationContext, int people_list_item, List<People> contactList) {
        super(applicationContext, people_list_item, contactList);
        filteredList =  contactList;
        getFilter();
    }
    @Override
    public int getCount() {
        return filteredList.size();
    }
    @Override
    public People getItem(int i) {
        return filteredList.get(i);
    }
    @Override
    public long getItemId(int i) {
        return i;
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        People peopleList = getItem(position);
        if (convertView == null) {
            convertView = LayoutInflater.from(getContext()).inflate(R.layout.people_list_item, null);
            holder = new ViewHolder();
            holder.firstName = (TextView) convertView.findViewById(R.id.firstName);
            holder.lastName = (TextView) convertView.findViewById(R.id.lastName);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.firstName.setText(peopleList.getFirstName());
        holder.lastName.setText(peopleList.getLastName());
        return convertView;
    }
    /**
     * Get custom filter
     * @return filter
     */
    @Override
    public Filter getFilter() {
        if (peopleFilter == null) {
            peopleFilter = new PeopleFilter();
        }
        return peopleFilter;
    }
    static class ViewHolder {
        TextView firstName;
        TextView lastName;
    }
    private class PeopleFilter extends Filter {
        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            FilterResults filterResults = new FilterResults();
            if (constraint!=null && constraint.length()>0) {
                ArrayList<People> tempList = new ArrayList<>();
                // search content in friend list
                for (People user : peopleList) {
                    if (user.getFirstName().toLowerCase().contains(constraint.toString().toLowerCase())) {
                        tempList.add(user);
                    }
                }
                filterResults.count = tempList.size();
                filterResults.values = tempList;
            } else {
                filterResults.count = peopleList.size();
                filterResults.values = peopleList;
            }
            return filterResults;
        }
        /**
         * Notify about filtered list to ui
         * @param constraint text
         * @param results filtered result
         */
        @SuppressWarnings("unchecked")
        @Override
        protected void publishResults(CharSequence constraint, Filter.FilterResults results) {
            filteredList = (ArrayList<People>) results.values;
            notifyDataSetChanged();
        }
    }
}
And this is PeopleListActivity
**PeopleListActivity** 
public class PeopleListActivity extends AppCompatActivity implements SearchView.OnQueryTextListener {
    private PeopleListFragment fragment;
    private FragmentManager displayFragment;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            Window window = getWindow();
            window.addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS);
            window.setStatusBarColor(Color.argb(255, 180, 110, 60));
        }
        setContentView(R.layout.activity_people);
        Toolbar toolbar = (Toolbar) findViewById(R.id.people_toolbar);
        setSupportActionBar(toolbar);
        toolbar.setNavigationIcon(R.drawable.ic_back_arrow);
        getSupportActionBar().setDisplayShowTitleEnabled(false);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(PeopleListActivity.this, NavigationActivity.class);
                startActivity(intent);
            }
        });
        displayFragment = getSupportFragmentManager();
        fragment = new PeopleListFragment();
        displayFragment.beginTransaction().add(R.id.people_fragment, fragment).commit();
        if (savedInstanceState == null) {
        }
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.options_menu, menu);
        SearchManager searchManager =(SearchManager) getSystemService(Context.SEARCH_SERVICE);
        SearchView searchView =(SearchView) menu.findItem(R.id.search).getActionView();
        searchView.setSearchableInfo(searchManager.getSearchableInfo(getComponentName()));
//        searchView.setIconifiedByDefault(false);
        searchView.setSubmitButtonEnabled(true);
        searchView.setOnQueryTextListener(this);
        return true;
//     return super.onCreateOptionsMenu(menu);
    }
    @Override
    public boolean onQueryTextSubmit(String query) {
        return false;
    }
    @Override
    public boolean onQueryTextChange(String newText) {
        PeopleAdapter peopleAdapter = (PeopleAdapter) PeopleListFragment.mContactlistView.getAdapter();
       peopleAdapter.getFilter().filter(newText);
        return true;
    }
}
Could anyone help me with this..?
 
     
     
    