第一回写android应用，第一次运行成功，第二次提示未知终止，拜托大家帮忙看看

第一次写android应用，第一次运行成功，第二次提示未知终止，拜托大家帮忙看看。
代码：

package com.studio.time;

import java.io.BufferedInputStream;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;

import org.apache.http.util.ByteArrayBuffer;
import org.apache.http.util.EncodingUtils;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.support.v4.app.NavUtils;
import android.view.View.OnClickListener;
import android.app.Activity;
import java.io.BufferedInputStream;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;
import org.apache.http.util.ByteArrayBuffer;
import org.apache.http.util.EncodingUtils;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

@SuppressLint({ "ParserError", "ParserError", "ParserError", "ParserError", "ParserError", "ParserError", "ParserError", "ParserError" })
public class MainActivity extends Activity {

private String TAG = "chapter8_3";
  private Button btnEnd;
  private TextView labelTimer;
  private Thread clockThread;
  private Thread urlThread;
  private boolean isRunning = true;
  private Handler handler;
  private TextView tv;


  @SuppressLint({ "ParserError", "ParserError" })
@Override
  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  btnEnd = (Button) findViewById(R.id.btnEnd);
  btnEnd.setOnClickListener(new OnClickListener() {

  public void onClick(View v) {
  isRunning = false;
  }
  });
   
  handler = new Handler() {

  public void handleMessage(Message msg) {
  switch (msg.what) {
  case 0:
  labelTimer.setText("you have been in this app " + msg.obj + " seconds");
  }
  }

  };

  labelTimer = (TextView) findViewById(R.id.labelTimer);
  tv = (TextView) findViewById(R.id.tv);
   
  /* 线程体是Clock对象本身，线程名字为"Clock" */
  clockThread = new Thread(new Runnable() {
  @SuppressLint("ParserError")
public void run() {
  int timer = 0;
  while (isRunning) {
  try {
  Thread.currentThread();
Thread.sleep(1000);
  timer++;
  /* labelTimer.setText("you have been in this app " + timer + " seconds"); */
  Message msg = new Message();
  msg.obj = timer;
  msg.what = 0;
  handler.sendMessage(msg);
  Log.d(TAG, "lost time " + timer);
  } catch (InterruptedException e) {
  e.printStackTrace();
  }
  }
  }