Check for null value before closing toServerWriter

src/main/java/ChatClient.java

@@ -77,8 +77,10 @@ public class ChatClient extends JFrame implements KeyListener {
                 }
             }
 
-            toServerWriter.close();
-
+            if(null != toServerWriter) {
+                toServerWriter.close();
+            }
+            
         }
     }