Android : at java.lang.String.

I am creating an android app in which i am sending data to the web service but i am getting this error

 java.lang.NullPointerException
  at java.lang.String.<init>(String.java:228

here is my code

for(int li=0;li<timerlist.size();li++)
            {
                String finish_pos=String.valueOf(li+1);
                int event_id=Integer.parseInt(series_event_raceid);
                 jObjectType.put("FinishPosition", finish_pos); 
                jObjectType.put("RaceTime", convertedtime.get(li));
                  jObjectType.put("SeriesEventRaceID", event_id);
                 jsonarraylist.add(jObjectType.toString());
                 }





 08-11 12:24:29.951: W/System.err(5262): java.lang.NullPointerException

08-11 12:24:29.951: W/System.err(5262): at java.lang.String.(String.java:228) 08-11 12:24:29.951: W/System.err(5262): at com.example1.racerunner.Timenewrace$5.onFailure(Timenewrace.java:394) 08-11 12:24:29.955: W/System.err(5262): at com.loopj.android.http.AsyncHttpResponseHandler.handleMessage(AsyncHttpResponseHandler.java:284) 08-11 12:24:29.959: W/System.err(5262): at com.loopj.android.http.AsyncHttpResponseHandler$ResponderHandler.handleMessage(AsyncHttpResponseHandler.java:134) 08-11 12:24:29.959: W/System.err(5262): at android.os.Handler.dispatchMessage(Handler.java:99) 08-11 12:24:29.959: W/System.err(5262): at android.os.Looper.loop(Looper.java:137) 08-11 12:24:29.959: W/System.err(5262): at android.app.ActivityThread.main(ActivityThread.java:5103) 08-11 12:24:29.959: W/System.err(5262): at java.lang.reflect.Method.invokeNative(Native Method) 08-11 12:24:29.959: W/System.err(5262): at java.lang.reflect.Method.invoke(Method.java:525) 08-11 12:24:29.959: W/System.err(5262): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 08-11 12:24:29.959: W/System.err(5262): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 08-11 12:24:29.963: W/System.err(5262): at dalvik.system.NativeStart.main(Native Method)