在一个EditText输入框中输入字符，activity被destroy之前通过onSaveInstanceState()保存输入字符，当重新create这个activity时，onCreate(Bundle)的Bundle参数却是null，不清楚问题出在哪？activity的代码如下：

package com.example.TestSaveInstanceState;

import android.app.Activity;
import android.os.Bundle;
import android.text.Editable;
import android.util.Log;
import android.widget.EditText;

public class MyActivity extends Activity {

    static final String STATE_KEY = "etInputState";
    EditText etInput;
    static final String MYTAG = "mytag";

    /**
     * Called when the activity is first created.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        etInput = (EditText)findViewById(R.id.etInput);
        if(savedInstanceState != null) {
            Log.w(MYTAG, savedInstanceState.getString(STATE_KEY));
            etInput.setText(savedInstanceState.getString(STATE_KEY));
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        String _s = etInput.getText().toString();
        if(_s != null) {
            outState.putString(STATE_KEY, _s);
            Log.w(MYTAG, "instance state = " + _s);
        }
    }
}