Facebook图形中出现错误-org.json.JSONException：名称无值

这段代码有什么错误？

我想添加一个Facebook页面名称并获取其json数据，但是我找不到一些错误。URL的这个例子，我用这个。这些是我使用的所有文件，并添加logcat的消息：

PagesActivity.java

package com.engahmedphp.facebookcollector;


import org.json.JSONException;
import org.json.JSONObject;

import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.DialogInterface;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class PagesActivity extends Activity {

    DatabaseHandler db = new DatabaseHandler(this);
    JSONObject json;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_pages);

        final Button button = (Button) findViewById(R.id.addPage);
        button.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                AlertDialog.Builder alert = new AlertDialog.Builder(
                        PagesActivity.this);

                alert.setTitle("Add New Page");
                alert.setMessage("Enter Page Name OR Valid Facebook Link");

                // Set an EditText view to get user input
                final EditText input = new EditText(PagesActivity.this);
                alert.setView(input);

                alert.setPositiveButton("Ok",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,
                                    int whichButton) {
                                String value = input.getText().toString();
                                // Do something with value!
                                String url = "http://graph.facebook.com/"
                                        + value + "/?fields=picture,name";                              
                                new MyAsyncTask().execute(url);

                            }
                        });

                alert.setNegativeButton("Cancel",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,
                                    int whichButton) {
                                // Canceled.
                            }
                        });

                alert.show();
            }
        });

    }

    private class MyAsyncTask extends AsyncTask<String, Void, Void> {

        ProgressDialog mProgressDialog;

        @Override
        protected void onPostExecute(Void result) {

            try {

                String name = json.getString("name");

                String fid = json.getString("id");
                String picture = json.getJSONObject("picture")
                        .getJSONObject("data").getString("url");

                Log.d("name", name);
                Log.d("fid", fid);
                Log.d("picture", picture);
                db.addPage(name, fid, picture);
            } catch (JSONException e) {
                e.printStackTrace();
            }

            mProgressDialog.dismiss();
        }

        @Override
        protected void onPreExecute() {

            mProgressDialog = ProgressDialog.show(PagesActivity.this,
                    "Loading...", "Data is Loading...");
        }

        @Override
        protected Void doInBackground(String... params) {           
                addPageData(params[0]);         
            return null;
        }
    }

    public JSONObject addPageData(String url) {

        // Creating JSON Parser instance
        JSONParser jParser = new JSONParser();

        // getting JSON string from URL
        json = jParser.getJSONFromUrl(url);

        return json;

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.splash, menu);
        return true;
    }

}

Logcat

08-31 08:21:35.972: W/System.err(8157): org.json.JSONException: No value for name
08-31 08:21:36.002: W/System.err(8157):     at org.json.JSONObject.get(JSONObject.java:354)
08-31 08:21:36.002: W/System.err(8157):     at org.json.JSONObject.getString(JSONObject.java:514)
08-31 08:21:36.002: W/System.err(8157):     at com.engahmedphp.facebookcollector.PagesActivity$MyAsyncTask.onPostExecute(PagesActivity.java:82)
08-31 08:21:36.002: W/System.err(8157):     at com.engahmedphp.facebookcollector.PagesActivity$MyAsyncTask.onPostExecute(PagesActivity.java:1)
08-31 08:21:36.012: W/System.err(8157):     at android.os.AsyncTask.finish(AsyncTask.java:631)
08-31 08:21:36.032: W/System.err(8157):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
08-31 08:21:36.032: W/System.err(8157):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
08-31 08:21:36.032: W/System.err(8157):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-31 08:21:36.062: W/System.err(8157):     at android.os.Looper.loop(Looper.java:137)
08-31 08:21:36.062: W/System.err(8157):     at android.app.ActivityThread.main(ActivityThread.java:5103)
08-31 08:21:36.062: W/System.err(8157):     at java.lang.reflect.Method.invokeNative(Native Method)
08-31 08:21:36.072: W/System.err(8157):     at java.lang.reflect.Method.invoke(Method.java:525)
08-31 08:21:36.072: W/System.err(8157):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
08-31 08:21:36.072: W/System.err(8157):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-31 08:21:36.072: W/System.err(8157):     at dalvik.system.NativeStart.main(Native Method)
08-31 08:21:36.072: I/Choreographer(8157): Skipped 72 frames!  The application may be doing too much work on its main thread.

我会错误地获取json对象吗？或不正确地使用异步或什么？