Wifi Scanning code

Here is my code:

package hu.wifx.droid;

import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.TextView;
import android.view.View;
import java.util.List;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.net.wifi.ScanResult;
import android.net.wifi.WifiManager;

public class WifXActivity extends Activity {

       TextView mText;
       WifiManager mWifi;
       List<ScanResult> wifiList;
       StringBuilder _sb = new StringBuilder();


   /** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.main);

       Button btn1 = (Button) findViewById(R.id.btn1);
       btn1.setOnClickListener(new View.OnClickListener()
       {
                       public void onClick(View arg0)
                       {
                               // TODO Auto-generated method stub
                               StartTester();
                       }
       });
   }

   public void StartTester()
   {
               IntentFilter i = new IntentFilter();
               i.addAction(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION);

               mText = (TextView) findViewById(R.id.mainText);
               mWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);
               registerReceiver(new BroadcastReceiver(){
                       public void onReceive(Context c, Intent i) {
                               WifiManager w = (WifiManager)c.getSystemService(Context.WIFI_SERVICE);
                               wifiList = w.getScanResults();

                               _sb = new StringBuilder();

                               for(int m = 0; m < wifiList.size(); m++)
                               {
                                       _sb.append((wifiList.get(m)).toString());
                                       _sb.append("\\n");
                               }

                               mText.setText(_sb);
                       }
               }, i);

               WifiManager WM = (WifiManager) getSystemService(Context.WIFI_SERVICE);
               WM.setWifiEnabled(true);
               WM.startScan();
         }
    }

No errors, but when I start the emulator and c开发者_运维知识库lick on button1:The application has stopped unexpectedly. Why?

Please check have you added these permissions in manifest:

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE"></uses-permission>
<uses-permission android:name="android.permission.CHANGE_WIFI_STATE"></uses-permission>