CheckBoxPreference isChecked导致FC在preferenceActivity中

public class preference extends PreferenceActivity implements OnSharedPreferenceChangeListener {

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


    @Override
    public void onCreate(Bundle savedInstanceState) {
        ...
        nwd_pref = (CheckBoxPreference)getPreferenceScreen().findPreference(nwd);
        ...
    }

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

findPrefernece(..)