Check connection to server before closing

src/main/java/ChatClient.java

@@ -61,10 +61,12 @@ public class ChatClient extends JFrame implements KeyListener {
         } catch (IOException e) {
             JOptionPane.showMessageDialog(null, "Verbindung zum Server \"" + address + "\" fehlgeschlagen.");
         } finally {
-            try {
-                connectionToServer.close();
-            } catch (IOException e) {
-                e.printStackTrace();
+            if (null != connectionToServer) {
+                try {
+                    connectionToServer.close();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
             }
 
             try {