Avoid closing fromServerReader in case of null

src/main/java/ChatClient.java

@@ -69,10 +69,12 @@ public class ChatClient extends JFrame implements KeyListener {
                 }
             }
 
-            try {
-                fromServerReader.close();
-            } catch (IOException e) {
-                e.printStackTrace();
+            if (null != fromServerReader) {
+                try {
+                    fromServerReader.close();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
             }
 
             toServerWriter.close();