NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference

Построил приложение для запроса и парсинга JSON, но не могу найти, где допустил ошибку. То ли в адаптере, то ли еще где-то.

FATAL EXCEPTION: main
  Process: com.example.harbinger95.requestbank, PID: 7584
  java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
    at android.widget.SimpleAdapter.getCount(SimpleAdapter.java:100)
    at android.widget.ListView.setAdapter(ListView.java:493)
    at android.app.ListActivity.setListAdapter(ListActivity.java:265)
    at com.example.harbinger95.requestbank.MainActivity$GetStudents.onPostExecute(MainActivity.java:90)
    at com.example.harbinger95.requestbank.MainActivity$GetStudents.onPostExecute(MainActivity.java:46)
    at android.os.AsyncTask.finish(AsyncTask.java:667)
    at android.os.AsyncTask.-wrap1(AsyncTask.java)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:684)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6121)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)

На List ничего не передается, но почему понять не могу.

MainActivity.java

package com.example.harbinger95.requestbank;
import android.app.ListActivity;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListAdapter;
import android.widget.SimpleAdapter;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.HashMap;
public class MainActivity extends ListActivity {
    // URL to get contacts JSON
    private static String url = "https://raw.githubusercontent.com/BilCode/AndroidJSONParsing/master/index.html";
    // JSON Node names
    private static final String TAG_STUDENT_INFO = "studentsinfo";
    private static final String TAG_ID = "id";
    private static final String TAG_STUDENT_NAME = "sname";
    private static final String TAG_EMAIL = "semail";
    private static final String TAG_ADDRESS = "address";
    private static final String TAG_STUDENT_GENDER = "gender";
    private static final String TAG_STUDENT_PHONE = "sphone";
    private static final String TAG_STUDENT_PHONE_MOBILE = "mobile";
    private static final String TAG_STUDENT_PHONE_HOME = "home";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
// Calling async task to get json
        new GetStudents().execute();
    }
    /**
     * Async task class to get json by making HTTP call
     */
    private class GetStudents extends AsyncTask<Void, Void, Void> {
        // Hashmap for ListView
        ArrayList<HashMap<String, String>> studentList;
        ProgressDialog proDialog;
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
// Showing progress loading dialog
            proDialog = new ProgressDialog(MainActivity.this);
            proDialog.setMessage("Please wait...");
            proDialog.setCancelable(false);
            proDialog.show();
        }
        @Override
        protected Void doInBackground(Void... arg0) {
// Creating service handler class instance
            WebRequest webreq = new WebRequest();
// Making a request to url and getting response
            String jsonStr = webreq.makeWebServiceCall(url, WebRequest.GETRequest);
            Log.d("Response: ", "> " + jsonStr);
            studentList = ParseJSON(jsonStr);
            return null;
        }
        @Override
        protected void onPostExecute(Void requestresult) {
            super.onPostExecute(requestresult);
// Dismiss the progress dialog
            if (proDialog.isShowing())
                proDialog.dismiss();
/**
 * Updating received data from JSON into ListView
 * */
            ListAdapter adapter = new SimpleAdapter(
                    MainActivity.this, studentList,
                    R.layout.item, new String[]{TAG_STUDENT_NAME, TAG_EMAIL,
                    TAG_STUDENT_PHONE_MOBILE}, new int[]{R.id.name,
                    R.id.email, R.id.mobile});
            setListAdapter(adapter);
        }
    }
    private ArrayList<HashMap<String, String>> ParseJSON(String json) {
        if (json != null) {
            try {
// Hashmap for ListView
                ArrayList<HashMap<String, String>> studentList = new ArrayList<HashMap<String, String>>();
                JSONObject jsonObj = new JSONObject(json);
// Getting JSON Array node
                JSONArray students = jsonObj.getJSONArray(TAG_STUDENT_INFO);
// looping through All Students
                for (int i = 0; i < students.length(); i++) {
                    JSONObject c = students.getJSONObject(i);
                    String id = c.getString(TAG_ID);
                    String name = c.getString(TAG_STUDENT_NAME);
                    String email = c.getString(TAG_EMAIL);
                    String address = c.getString(TAG_ADDRESS);
                    String gender = c.getString(TAG_STUDENT_GENDER);
// Phone node is JSON Object
                    JSONObject phone = c.getJSONObject(TAG_STUDENT_PHONE);
                    String mobile = phone.getString(TAG_STUDENT_PHONE_MOBILE);
                    String home = phone.getString(TAG_STUDENT_PHONE_HOME);
// tmp hashmap for single student
                    HashMap<String, String> student = new HashMap<String, String>();
// adding every child node to HashMap key => value
                    student.put(TAG_ID, id);
                    student.put(TAG_STUDENT_NAME, name);
                    student.put(TAG_EMAIL, email);
                    student.put(TAG_STUDENT_PHONE_MOBILE, mobile);
// adding student to students list
                    studentList.add(student);
                }
                return studentList;
            } catch (JSONException e) {
                e.printStackTrace();
                return null;
            }
        } else {
            Log.e("ServiceHandler", "No data received from HTTP Request");
            return null;
        }
    }
}

Webrequest.java

package com.example.harbinger95.requestbank;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.HashMap;
import java.util.Map;
import javax.net.ssl.HttpsURLConnection;
/**
 * Created by harbinger95 on 16.05.17.
 */
public class WebRequest {
    static String response = null;
    public final static int GETRequest = 1;
    public final static int POSTRequest = 2;
    //Constructor with no parameter
    public WebRequest() {
    }
    /**
     * Making web service call
     *
     * @url - url to make web request
     * @requestmethod - http request method
     */
    public String makeWebServiceCall(String url, int requestmethod) {
        return this.makeWebServiceCall(url, requestmethod, null);
    }
    /**
     * Making web service call
     *
     * @url - url to make web request
     * @requestmethod - http request method
     * @params - http request params
     */
    public String makeWebServiceCall(String urladdress, int requestmethod,
                                     HashMap<String, String> params) {
        URL url;
        String response = "";
        try {
            url = new URL(urladdress);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(15001);
            conn.setConnectTimeout(15001);
            conn.setDoInput(true);
            conn.setDoOutput(true);
            if (requestmethod == POSTRequest) {
                conn.setRequestMethod("POST");
            } else if (requestmethod == GETRequest) {
                conn.setRequestMethod("GETRequest");
            }
            if (params != null) {
                OutputStream ostream = conn.getOutputStream();
                BufferedWriter writer = new BufferedWriter(
                        new OutputStreamWriter(ostream, "UTF-8"));
                StringBuilder requestresult = new StringBuilder();
                boolean first = true;
                for (Map.Entry<String, String> entry : params.entrySet()) {
                    if (first)
                        first = false;
                    else
                        requestresult.append("&");
                    requestresult.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
                    requestresult.append("=");
                    requestresult.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
                }
                writer.write(requestresult.toString());
                writer.flush();
                writer.close();
                ostream.close();
            }
            int reqresponseCode = conn.getResponseCode();
            if (reqresponseCode == HttpsURLConnection.HTTP_OK) {
                String line;
                BufferedReader br = new BufferedReader(new InputStreamReader(conn.getInputStream()));
                while ((line = br.readLine()) != null) {
                    response += line;
                }
            } else {
                response = "";
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return response;
    }
}

Activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <ListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</RelativeLayout>

item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    <!-- Name Label -->
    <TextView
        android:id="@+id/name"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:paddingBottom="2dip"
        android:paddingTop="6dip"
        android:text="Name"
        android:textSize="16sp"
        android:textStyle="bold" />
    <!-- Email label -->
    <TextView
        android:id="@+id/email"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:paddingBottom="2dip" />
    <!-- Mobile number label -->
    <TextView
        android:id="@+id/mobile"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="left"
        android:text="Mobile: " />
</LinearLayout>