android - NPE when try to contact with saved data -

i have nullpointerexception when try saved data.i think caused wrong "open" method @ sqlhelper class.and associated wrong context.because when used activity worked,now use fragment , have error. error:

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:     @ com.example.vnvbnv.myapplication.mainlist.displaysavedlv(mainlist.java:168) 10-10 09:23:03.009 7657-7657/com.example.vnvbnv.myapplication e/androidruntime:     @ com.example.vnvbnv.myapplication.mainlist.onactivitycreated(mainlist.java:56) 

mainlist(lines caused error i'll mark "****") :

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();***** , 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 =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 no active networks.             return false;         } else             return true;     } } 

and open method @ sqlhelper , constructor(i think problem here):

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;     }