CheckBoxPreference isChecked causes a FC in a preferenceActivity

When I check if the checkbox is checked, I get a Force Close in the following code:

public class preference extends PreferenceActivity implements OnSharedPreferenceChangeListener {

    public static final String nwd = "nwd";
    private CheckBoxPreference nwd_pref;


    @Override
    publi开发者_如何学编程c void onCreate(Bundle savedInstanceState) {
        ...
        nwd_pref = (CheckBoxPreference)getPreferenceScreen().findPreference(nwd);
        ...
    }

    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
    ...
    if (nwd_pref.isChecked()){
       // do code
    }
    ...
    }
}

I found this example during searching but looks the same to me.

Thanks for your help!

---

findPrefernece(..) will "Return The Preference with the key, or null." It looks like "nwd" hasn't been set yet.