我有做2個頁面，一個是目錄頁面，一個是登入頁面
我想讓使用者看看目錄，若要購買(按下購買鍵)
他就會跳到登入頁面，若發現她已經登入，就可直接訂購

但現在我發現，我這樣做都會當掉
MainActivity.java

public class MainActivity extends Activity {
   ArrayList<Team> teams = new ArrayList<Team>();
    ListView listview;
    String account="";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

        Intent intent=this.getIntent();
        Bundle bundle=intent.getExtras();
        if(!bundle.getString("ACCOUNT").equals(null) ){
        //加入這行就會當機，可是我有先判斷他是不是null了耶....
            account=bundle.getString("ACCOUNT");
           }
     listview = (ListView) findViewById(R.id.listview);
     listview.setOnItemClickListener(lvonclick);
    Team team =new Team("my_product","price");
     teams.add(team);
      team = new Team("fish", "77");//產品
      teams.add(team); 
          final TeamsAdapter adapter = new TeamsAdapter(this, R.layout.team, teams);
            listview.setAdapter(adapter);
   }

         private ListView.OnItemClickListener lvonclick=
                    new ListView.OnItemClickListener(){
                    @Override
                    public void onItemClick(AdapterView<?> parent, View v,
                            int position, long id) {
                        String sel=parent.getItemAtPosition(position).toString();   
                     mydialog(sel);

                    }
                };

private void mydialog(String str1){
    new AlertDialog.Builder(MainActivity.this)
    .setTitle("product")
    .setIcon(R.drawable.ic_launcher)
    .setMessage(str1)
    .setPositiveButton("buy", new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialoginterface, int i)
        {
            if(account==""){
                Intent intent2=new Intent();//jump to login page
            intent2.setClass(MainActivity.this,Login.class);
            startActivity(intent2);
            }
        }
    })
    .show(); 
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
  getMenuInflater().inflate(R.menu.main, menu);
    return true;
}
}

login.java

public class Login extends Activity {//登入畫面
 private TextView acc,pwd;
 private  Button  btn;
 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.login);
      StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()             
     acc=(TextView)findViewById(R.id.acc);
     pwd=(TextView)findViewById(R.id.pwd);
     btn=(Button)findViewById(R.id.btn);
     btn.setOnClickListener(btnlogin);


}

private Button.OnClickListener btnlogin=new Button.OnClickListener(){
    public void onClick(View v){
        String account=acc.getText().toString();
        String passwd=pwd.getText().toString();
            Intent intent= new Intent();
                intent.setClass(Login.this, MainActivity.class);
                Bundle bundle=new Bundle();
                if(account=="123")
                {
                  bundle.putString("ACCOUNT", account);
                  intent.putExtras(bundle);
                  startActivity(intent);
                }
    }
};
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}
}

錯誤訊息:

請大家指教，該如何修改，謝謝