在C#winform中断开连接后无法重新连接服务器

我正在做一个应用程序，它要求我作为客户端连接到服务器并从服务器接收数据。当连接断开时，我需要重新连接。以下是我的代码：

   public enum MySocketState
   {
       Disconnected = 0,
       Connecting,
       Connected
   }
    private Socket _serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
    private byte[] _recieveBuffer = new byte[128];
    void DataProcess()
    {
        try
        {
            if (_serverSocket == null || sockState == MySocketState.Disconnected)
            {
                Console.WriteLine("Trying to connect...");
                SetupServer();
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
    }
    private void SetupServer()
    {
        try
        {
            _serverSocket.Connect("192.168.1.11", 1000);
        }
        catch (SocketException ex)
        {
            Console.WriteLine(ex.Message);
        }
        sockState = MySocketState.Connected;
        Console.WriteLine("Server connected...");
        _serverSocket.BeginReceive(_recieveBuffer, 0, _recieveBuffer.Length, SocketFlags.None, new AsyncCallback(ReceiveCallback), null);
    }
    private void ReceiveCallback(IAsyncResult AR)
    {
        try
        {
            if (_serverSocket == null)
                _serverSocket = (Socket)AR.AsyncState;
            int recieved = _serverSocket.EndReceive(AR);
            if (recieved <= 0)
            {
                CloseSocket();
                return;
            }
            byte[] recData = new byte[recieved];
            Buffer.BlockCopy(_recieveBuffer, 0, recData, 0, recieved);
            string strData = ASCIIEncoding.ASCII.GetString(recData);
            Console.WriteLine(strData);
            //Process the data stored in recData
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
            CloseSocket();
        }
        finally
        {
            try
            {
                //Start receiving again
                if (_serverSocket != null)
                    _serverSocket.BeginReceive(_recieveBuffer, 0, _recieveBuffer.Length, SocketFlags.None, new AsyncCallback(ReceiveCallback), null);
            }
            catch (Exception ex2)
            { }
        }
    }
    public void CloseSocket()
    {
        try
        {
            if (_serverSocket != null)
            {
                if (_serverSocket.Connected)
                    _serverSocket.Shutdown(SocketShutdown.Both);
                _serverSocket.Close();
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
        _serverSocket = null;
        sockState = MySocketState.Disconnected;
    }
    private void timer1_Tick(object sender, EventArgs e)
    {
        DataProcess();
    }

我使用计时器（timer1_tick）来检测服务器是否已连接。

但在它与服务器断开连接后，它无法重新连接，下面是_serverSocket.Connect("192.168.1.11", 1000):线上的错误消息

A first chance exception of type 'System.NullReferenceException' occurred in fform.exe
System.NullReferenceException: Object reference not set to an instance of an object.
at fform.Form1.SetupServer() in c:'Work'Project 2015'Form1.cs:line 143
at fform.Form1.DataProcess() in c:'Work'Project 2015'Form1.cs:line 79

你们知道为什么不能连接回来吗？

public void CloseSocket()
{
    ...
    _serverSocket = null;
    ...
}

private void SetupServer()
{
    try
    {
        _serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp); //Added
        _serverSocket.Connect("192.168.1.11", 1000);
    }
    ...
}