多线程 未将对象引用设置到对象的实例,该如何处理

多线程 未将对象引用设置到对象的实例
我用多个线程调用带参数的函数 
初始化： 
   if (recvThread1==null)
            {
                recvThread1 = new Thread(new ParameterizedThreadStart(ReceiveData));
                handleThread1=new Thread(new ParameterizedThreadStart(HandleRecvData));
            }
           else if (recvThread2 == null) 
           {
               recvThread2 = new Thread(new ParameterizedThreadStart(ReceiveData));
               handleThread2 = new Thread(new ParameterizedThreadStart(HandleRecvData));
           }

后边调用：
recvThread1.Start(port1);
recvThread2.Start(port2);

运行后这条报错：recvThread2.Start(port2);———>未将对象引用设置到对象的实例

为什么啊...
[解决办法]
 else if (recvThread2 == null) 
=>
if (recvThread2 == null)