APP INACTIVE或按下主页按钮5分钟后结束会话 - android

Question

当android APP INACTIVE或按下主页按钮后，如何在（注销）5分钟后编码结束会话？我使用下面的代码......用于保存数据保存................................. ....................................

    public class LogoutService extends Service {
    public static CountDownTimer timer;

    @Override
    public void onCreate() {
        // TODO Auto-generated method stub
        super.onCreate();
        timer = new CountDownTimer(1, 1000) {//1 * 60 * 1000, 1000
            public void onTick(long millisUntilFinished) {
                //Some code
               // Log.v(Constants.TAG, "Service Started");
            }

            public void onFinish() {
              // Log.v(SyncStateContract.Constants.TAG, "Call Logout by Service");
                // Code for Logout
                dialogBox();
                Log.i("fr","fr");

                stopSelf();
            }
        };
    }
    public void dialogBox() {
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
        alertDialogBuilder.setMessage("Your session has expired.");
        alertDialogBuilder.setPositiveButton("Ok",
                new DialogInterface.OnClickListener() {



                   @Override
                    public void onClick(DialogInterface arg0, int arg1) {
                    }
                });

    //        alertDialogBuilder.setNegativeButton("cancel",
    //                new DialogInterface.OnClickListener() {
//

    //                    @Override
//                    public void onClick(DialogInterface arg0, int arg1) {
//
//                    }
//                });

         AlertDialog alertDialog = alertDialogBuilder.create();
        alertDialog.show();
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        return null;



 }
}

on MainActivity：

@Override
protected void onResume() {
    // TODO Auto-generated method stub
    super.onResume();

    LogoutService.timer.start();
}

@Override
protected void onStop() {
    // TODO Auto-generated method stub
    super.onStop();
    LogoutService.timer.cancel();
} 

错误：

3-28 01:52:12.381 1250-1250/? E/AndroidRuntime: FATAL EXCEPTION: main
                                                 Process: com. . ., PID: 1250
                                                 java.lang.NullPointerException