android - Application crushes and throw NullPointerException while Orientation is changing -

i using fragment , put asynctask inside network oriented task. works fine. when change orientation protrait landscape, wors fine. when changed orientation again portrait, crashes. trying code.

public class problemstatistics extends sherlockprogressfragment {     view mcontentview;      @override     public view oncreateview(layoutinflater inflater, viewgroup container,             bundle savedinstancestate) {         mcontentview = inflater.inflate(r.layout.submission_statistics,                 container, false);         return super.oncreateview(inflater, container, savedinstancestate);     }      @override     public void onactivitycreated(bundle savedinstancestate) {         super.onactivitycreated(savedinstancestate);         setcontentshown(false);         setcontentview(mcontentview);         bundle bundle = this.getarguments();         int problemno = 100;         if (bundle != null) {             problemno = bundle.getint("problem_no", problemno);         }         new getproblemstatistictask().execute(commonutils.specific_problem_url + problemno);     }      @override     public void onstop() {         ((mainactivity) getsherlockactivity()).setproblemstatistics(false);         super.onstop();     }      protected class getproblemstatistictask extends             asynctask<string, void, inputstreamreader> {          @override         protected inputstreamreader doinbackground(string... params) {             return new jsondownloader().getjsonstringfromurl(params[0]);         }          @override         protected void onpostexecute(inputstreamreader isr) {             super.onpostexecute(isr);             // other stuff , parsing related code             layout.addview(gview);             setcontentshown(true);         }     }  } 

and error log:

08-16 00:07:18.025: e/androidruntime(542): java.lang.nullpointerexception 08-16 00:07:18.025: e/androidruntime(542):  @ me.kaidul.uhunt.latestsubmissions$getsubmissionslisttask.onpostexecute(latestsubmissions.java:84) 08-16 00:07:18.025: e/androidruntime(542):  @ me.kaidul.uhunt.latestsubmissions$getsubmissionslisttask.onpostexecute(latestsubmissions.java:1) 08-16 00:07:18.025: e/androidruntime(542):  @ android.os.asynctask.finish(asynctask.java:417) 08-16 00:07:18.025: e/androidruntime(542):  @ android.os.asynctask.access$300(asynctask.java:127) 08-16 00:07:18.025: e/androidruntime(542):  @ android.os.asynctask$internalhandler.handlemessage(asynctask.java:429) 08-16 00:07:18.025: e/androidruntime(542):  @ android.os.handler.dispatchmessage(handler.java:99) 08-16 00:07:18.025: e/androidruntime(542):  @ android.os.looper.loop(looper.java:123) 08-16 00:07:18.025: e/androidruntime(542):  @ android.app.activitythread.main(activitythread.java:4627) 08-16 00:07:18.025: e/androidruntime(542):  @ java.lang.reflect.method.invokenative(native method) 08-16 00:07:18.025: e/androidruntime(542):  @ java.lang.reflect.method.invoke(method.java:521) 08-16 00:07:18.025: e/androidruntime(542):  @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:868) 08-16 00:07:18.025: e/androidruntime(542):  @ com.android.internal.os.zygoteinit.main(zygoteinit.java:626) 08-16 00:07:18.025: e/androidruntime(542):  @ dalvik.system.nativestart.main(native method) 

when orientation of phone changes, activity destroyed , created again.

you can use in androidmanifest.xml.

activity    activity...    android:configchanges="orientation|keyboardhidden|screensize">  activity 

edited:

as complementary answer:

the documentations says:"a fragment must embedded in activity , fragment's lifecycle directly affected host activity's lifecycle. example, when activity paused, fragments in it, , when activity destroyed, fragments."

here link http://developer.android.com/guide/components/fragments.html

you can't recreate activity fragments, should rethink approach.