android - Jsoup Nullpointer 例外

Question

エラーログ：

04-16 23:05:29.133: W/dalvikvm(7474): threadid=1: thread exiting with uncaught exception (group=0x41550930)
04-16 23:05:29.133: E/AndroidRuntime(7474): FATAL EXCEPTION: main
04-16 23:05:29.133: E/AndroidRuntime(7474): java.lang.NullPointerException
04-16 23:05:29.133: E/AndroidRuntime(7474):     at com.example.yo.MainActivity$jsoupexample.onPostExecute(MainActivity.java:74)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at com.example.yo.MainActivity$jsoupexample.onPostExecute(MainActivity.java:1)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at android.os.AsyncTask.finish(AsyncTask.java:631)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at android.os.Looper.loop(Looper.java:137)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at android.app.ActivityThread.main(ActivityThread.java:5041)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at java.lang.reflect.Method.invokeNative(Native Method)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at java.lang.reflect.Method.invoke(Method.java:511)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
04-16 23:05:29.133: E/AndroidRuntime(7474):     at dalvik.system.NativeStart.main(Native Method)
04-16 23:05:29.163: D/dalvikvm(7474): GC_CONCURRENT freed 557K, 9% free 8027K/8736K, paused 2ms+4ms, total 28ms

コード

@TargetApi(Build.VERSION_CODES.GINGERBREAD)
public class MainActivity extends Activity {

    Document doc;
    TextView tv;
    String res="";
    String BLOG_URL = "http://www.google.co.in/";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        // set layout view
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new jsoupexample().execute(BLOG_URL);
    }
    public class jsoupexample extends AsyncTask<String,Integer,String>
    {
        @Override
        protected String doInBackground(String... BLOG_URL) {
            try {
                doc= Jsoup.connect(BLOG_URL[0]).userAgent("Mozilla/5.0").timeout(20000).get();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            Elements links = doc.select("a[href]");
            for(Element element: links)
            { 
                res = element.attr("href");
            }
            return res;
        }
        @Override
        protected void onPostExecute(String result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);

            if(result!=null && !result.isEmpty())
            tv.setText(result);
            else
            tv.setText("no value returned");
        }
    }   
}