尝试与保存的数据联系时的NPE

Question

当我尝试获取我保存的数据时，我有一个NullPointerException。我认为它在我的SqlHelper类中导致错误的“打开”方法。它与错误的上下文相关联。因为当我使用Activity时所有工作，现在我使用Fragment并且有这个错误。 错误：

10-10 09:23:03.009 7657-7657/com.example.vnvbnv.myapplication E/AndroidRuntime:  Caused by: java.lang.NullPointerException
10-10 09:23:03.009 7657-7657/com.example.vnvbnv.myapplication E/AndroidRuntime:     at com.example.vnvbnv.myapplication.MainList.displaysavedlv(MainList.java:168)
10-10 09:23:03.009 7657-7657/com.example.vnvbnv.myapplication E/AndroidRuntime:     at com.example.vnvbnv.myapplication.MainList.onActivityCreated(MainList.java:56)

MainList（导致错误的行我将用“****”标记）：

public class MainList extends ListFragment{
    SqlHelper dbHelper;
    ListView mainList;
    ProgressBar progBar;
    private static String url = "https://fierce-citadel-4259.herokuapp.com/hamsters";
    private static final String TITLE = "title";
    private static final String DESCRIPTION = "description";
    private static final String IMAGE = "image";
    ArrayList<HashMap<String,String>> jsonlist1 = new ArrayList<HashMap<String, String>>();
    ArrayList<HashMap<String,String>> bdList = new ArrayList<HashMap<String, String>>();
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.list_fragme, null);
        mainList = (ListView)v.findViewById(android.R.id.list);
progBar = (ProgressBar) v.findViewById(R.id.progressBar);
        return v;
    }




    public void onActivityCreated(Bundle savedInstanceState){
        super.onActivityCreated(savedInstanceState);
if (isNetworkConnected()==true) {
    new ProgressTask().execute();
}
        else{
    displaysavedlv();***** and this!
}
    }

    @Override
    public void onListItemClick(ListView l, View view, int position, long id) {
        super.onListItemClick(l, view, position, id);
        String title = jsonlist1.get(position).get("title");
        String description= jsonlist1.get(position).get("description");
        String image = jsonlist1.get(position).get("image");


        MyDetailFragment detailFragment = new MyDetailFragment();
        Bundle bundle = new Bundle();
        if(isNetworkConnected()==true) {
            bundle.putString("title", title);
            bundle.putString("description", description);
            bundle.putString("image", image);
        }
        else
        {
            String dbtitle= bdList.get(position).get("title");
            String dbdescription = bdList.get(position).get("description");
            String dvimage = bdList.get(position).get("image");
            bundle.putString("title",dbtitle);
            bundle.putString("description",dbdescription);
            bundle.putString("image",dvimage);
        }
        detailFragment.setArguments(bundle);
        FragmentTransaction fragmentTransaction = getActivity().getFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.FragmentCont,detailFragment);
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();
    }

    private class ProgressTask extends AsyncTask<String,Void,Boolean> {
        private ProgressDialog dialog;
        private Activity activity;
        private MainActivity context;
        private String[] params;

        public ProgressTask(MainActivity activity) throws SQLException {
            this.activity = getActivity();
            context = activity;
            dialog = new ProgressDialog(getActivity().getApplicationContext());
dbHelper = new SqlHelper(getActivity().getApplicationContext());
        }

        public ProgressTask()  {
            dialog = new ProgressDialog(getActivity().getApplicationContext());
            try {
                dbHelper = new SqlHelper(getActivity().getApplicationContext());
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        @Override
        protected Boolean doInBackground(String... params) {
            this.params = params;
            JSONParser jParser = new JSONParser();
            JSONArray json = jParser.getJSONFromUrl(url);
            for(int i =0;i<json.length();i++) {
                try {
                    JSONObject c = json.getJSONObject(i);
                    String vtitle = c.getString(TITLE);
                    String vdescription = c.getString(DESCRIPTION);
                    String vimage = c.getString(IMAGE);
                    dbHelper.open();
                    dbHelper.createEntry(vtitle, vimage, vdescription);
                    dbHelper.close();

                    HashMap<String, String> map = new HashMap<>();
                    map.put(TITLE, vtitle);
                    map.put(DESCRIPTION, vdescription);
                    map.put(IMAGE, vimage);

                    jsonlist1.add(map);

                } catch (JSONException e) {
                    e.printStackTrace();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            return null;
        }


        protected void onPreExecute(){
progBar.setVisibility(View.VISIBLE);
        }
        protected void onPostExecute(final Boolean success){



if (isNetworkConnected()==true) {
    progBar.setVisibility(View.GONE);
    CustomListAdapter adapter = new CustomListAdapter(getActivity(), jsonlist1, R.layout.list_item, new String[]{TITLE, DESCRIPTION}, new int[]{R.id.title, R.id.description});
    mainList.setAdapter(adapter);
}
            else {
    displaysavedlv();
}




        }

    }
    private void displaysavedlv() {
        bdList = dbHelper.getAllData();****(this one)

        CustomListAdapter adapter1 = new CustomListAdapter(getActivity(), bdList, R.id.list_item, new String[]{TITLE, DESCRIPTION}, new int[]{R.id.title, R.id.description});
        mainList.setAdapter(adapter1);
    }
    private boolean isNetworkConnected() {
        ConnectivityManager cm = (ConnectivityManager)getActivity().getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo ni = cm.getActiveNetworkInfo();
        if (ni == null) {
            // There are no active networks.
            return false;
        } else
            return true;
    }
}

和我在SqlHelper和构造函数中的open方法（我觉得问题在这里）：

public SqlHelper(Context c) throws SQLException {
        ourContext = c;
        try {
            open();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public SqlHelper open() throws SQLException{
        ourHelper = new DbHelper(ourContext);
        ourDatabase = ourHelper.getWritableDatabase();
        return this;
    }