Memory leak caused by handler (case one)

The code
that causes the memory leak is as follows: I believe everyone can see it and wrote some pseudo-code

Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            //todo 这里做一些事情。。。
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        new Thread(new Runnable() {
            @Override
            public void run() {
                SystemClock.sleep(3000);
                Message msg = new Message();
                msg.what = 3;
                handler.sendMessage(msg);
            }
        }).start();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
    }

In this case, when you exit the software, the thread is executing and still holds a reference to the activity, which will cause a memory leak

Solution code: It is not weak drinking, please refer to it,

    Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            //todo 这里做一些事情。。。
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        new Thread(new Runnable() {
            @Override
            public void run() {
                SystemClock.sleep(3000);
                Message msg = new Message();
                msg.what = 3;
                //这里是修改点
                if (handler != null){
                    handler.sendMessage(msg);
                }
            }
        }).start();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        //注意这里是修改点
        handler.removeCallbacksAndMessages(null);
        handler = null;
		//第二种写法 把message 全局变量
		//message.recycle();
    }

Why write it like this?