Null Reference Exception?

Sup /prog/

Im trying to create a multi-client server in C#. I'm making it access an ArrayList of the same class. (I just started C# a week ago and are trying to gain experience)

Anyways, once it gets to the clientThread (I pass the index of the ArrayList in which is contained TcpClient). I get a nullreference exception.

Why?

>>18
I've had experience with MSDN before and I can assure you that it is a very helpful resource when you have to write some shitty academic fib calculator, but is completely useless in real world.