FATAL EXCEPTION: AsyncTask #1 RuntimeException: An error occured while executing doInBackground()

Question

I am accepting address by user and then trying to get division_id and division_name for that i am using AsyncTask.

Every time i am getting : RuntimeException: An error occured while executing doInBackground()

And facing exception at this line: Caused by: java.lang.NullPointerException at com.test.myapplication.YieldFragment$GetHttpResponse.doInBackground(YieldFragment.java:158) -- > is refer to collegeList.add(map); at com.test.myapplication.YieldFragment$GetHttpResponse.doInBackground(YieldFragment.java:117) --> is refer to private class GetHttpResponse extends AsyncTask {

Logcat:

09-18 02:09:25.644 2118-2140/? E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
                                             Process: com.test.myapplication, PID: 2118
                                             java.lang.RuntimeException: An error occured while executing doInBackground()
                                                 at android.os.AsyncTask$3.done(AsyncTask.java:300)
                                                 at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
                                                 at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
                                                 at java.util.concurrent.FutureTask.run(FutureTask.java:242)
                                                 at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
                                                 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
                                                 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
                                                 at java.lang.Thread.run(Thread.java:841)
                                              Caused by: java.lang.NullPointerException
                                                 at com.test.myapplication.YieldFragment$GetHttpResponse.doInBackground(YieldFragment.java:158)
                                                 at com.test.myapplication.YieldFragment$GetHttpResponse.doInBackground(YieldFragment.java:117)
                                                 at android.os.AsyncTask$2.call(AsyncTask.java:288)
                                                 at java.util.concurrent.FutureTask.run(FutureTask.java:237)
                                                 at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231) 
                                                 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112) 
                                                 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587) 
                                                 at java.lang.Thread.run(Thread.java:841) 

code:

private class GetHttpResponse extends AsyncTask<Void, Void, Void> {
    private Context context;
    String result;
    List<HashMap<String, String>> collegeList;


    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected Void doInBackground(Void... arg0) {
        List<Estate> estates = new LinkedList<Estate>();
        dbHelper.getReadableDatabase();
        estates = dbHelper.getAllEstate();


        //Syntax for sort Listview Array by Id Ascending
        Collections.sort(estates, new Comparator<Estate>() {
            @Override
            public int compare(Estate id1, Estate id2) {
                return id1.getDivision_id() - id2.getDivision_id();
            }
        });

        // looping through All data

        for (Estate temp_vg : estates) {
            temp_vg.getDivision_id();
            temp_vg.getDivision_name();

            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();

           // map = new LinkedList<Estate>();
            // adding each child node to HashMap key => value
            map.put("division_id", String.valueOf(temp_vg.getDivision_id()));
            map.put("division_name", temp_vg.getDivision_name());

            // adding HashList to ArrayList
            collegeList.add(map);
        }

        return null;
    }

    @Override
    protected void onPostExecute(Void result)

    {
        proCollageList.setVisibility(View.GONE);
        listCollege.setVisibility(View.VISIBLE);
        if (collegeList != null) {

            //adapter = new ListAdapterEstate(LihatDataEstate.this, map);
            ListAdapterEstate adapter = new ListAdapterEstate(collegeList, context);
            listCollege.setAdapter(adapter);

            //adapter=new LazyAdapter(Lihat_Data.this, dataList);
            //list.setAdapter(adapter);

        }else {
            Toast.makeText(getActivity(), "No Data Estate Available !", Toast.LENGTH_LONG).show();
        }
    }
}

}