无法在我的侦听器和广播接收器中涉及接口方法空对象引用

也许我的问题标题在stackoverflow上还有其他问题，但是我找不到任何有用的信息，所以我需要任何人帮助解决我的特定问题：我想连续检查Internet连接，因此我创建了一个广播接收器扩展类，它具有侦听器如下：

NetworkStateReceiver.java：

public class NetworkStateReceiver extends BroadcastReceiver {

    public interface NetworkStateListener {
        void onNetChange(int status);
    }

    private NetworkStateListener mOnNetworkStateListener;

    public void setNetworkStateListener(NetworkStateListener listener) {
        mOnNetworkStateListener = listener;
    }

    public void onReceive(Context context, Intent intent) {
        int status = NetworkUtil.getConnectivityStatus(context);
        if (mOnNetworkStateListener != null)
            mOnNetworkStateListener.onNetChange(status);
    }
}

在我的活动中：

public class FrgChoosing extends Fragment {
    private NetworkStateReceiver mNetworkStateCallback;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {   
        ...
        initNetworkChecking();
    } 
    private void initNetworkChecking() {
        mNetworkStateCallback = new NetworkStateReceiver();
        mNetworkStateCallback.setNetworkStateListener(new NetworkStateListener() {      
            @Override
            public void onNetChange(int status) {
                // TODO Auto-generated method stub
                switch (status) {
                case NetworkUtil.TYPE_WIFI:
                    Functions.toastString("have wifi connection", getActivity());
                    break;
                case NetworkUtil.TYPE_MOBILE:
                    Functions.toastString("have 3g connection", getActivity());
                    break;
                case NetworkUtil.TYPE_NOT_CONNECTED:
                    Functions.toastString("no connection", getActivity());
                    break;
                default:
                    break;
                }
            }
        });
    }
}

在清单中：

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
...
<receiver android:name=".NetworkStateReceiver">
    <intent-filter>
        <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
        <action android:name="android.net.wifi.WIFI_STATE_CHANGED" />
    </intent-filter>
</receiver>

在看到logcat之后，我相信当触发NetworkStateReceiver.onReceiver时，mOnNetworkStateListener始终为null。我不知道错误在哪里？有人可以帮助我解决此问题吗？预先感谢大家。