Java NullPointerException: "this.writer" is null in ClientWindow.actionPerformed()

The exception is a 'NullPointerException' and is caused by trying to invoke the 'println()' method on a 'PrintWriter' object that is 'null'. The error occurs on line 63 of the 'ClientWindow' class in the 'actionPerformed()' method. The 'this.writer' object has not been initialized or assigned a value, so it is 'null'. To fix the error, the 'this.writer' object should be initialized before trying to use it.