空指针异常问题

public class UdpClientBiz {

    //服务器IP
    private String mServerIp = "192.168.95.2";
    //服务器端口号
    private int mServerPort = 7777;
    //服务器的InetAddress
    private InetAddress mServerAddress;
    //DatagramSocket
    private DatagramSocket datagramSocket;

    public UdpClientBiz() {
        try {
            //通过服务器IP获取服务器的InetAddress对象
            mServerAddress = InetAddress.getByName(mServerIp);
            //实例化DatagramSocket对象
            this.datagramSocket = new DatagramSocket();
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }

    //用来处理服务器返回的数据
    public interface OnReturnedMsgListener {
        void handleMsg (String msg);
    }

    //向服务器发送、接收数据
    public void sendAndReceiveMsg (final String msg, final OnReturnedMsgListener listener) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    //发送数据到服务器
                    byte[] clientMsgBytes = msg.getBytes();
                    DatagramPacket clientPacket = new DatagramPacket(clientMsgBytes, clientMsgBytes.length,
                            mServerAddress, mServerPort);
                    datagramSocket.send(clientPacket);
                    //从服务器接收数据
                    byte[] buf = new byte[1024];
                    DatagramPacket serverMsgPacket = new DatagramPacket(buf, 0, buf.length);
                    datagramSocket.receive(serverMsgPacket);
                    //处理从服务器接收的数据
                    listener.handleMsg(new String(serverMsgPacket.getData()));
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }
}

在执行

datagramSocket.send(clientPacket);

的时候报了一个DatagramSocket的空指针异常，为何？