Exception in thread "Thread-0" java.lang.NullPointerException: Cannot invoke "java.net.ServerSocket.close()" because "this.sock" is null

[ParoneQWERTY]

Hi, I get an error whenever I try press the "Listen" button in the server GUI. I looked around the code and found out its because of the server socket was null. I have only just recently gotten into networking so I don't really know what to do. Thanks!