Check connection to server before closing

src/main/java/ChatClient.java

@@ -61,11 +61,13 @@ public class ChatClient extends JFrame implements KeyListener {
         } catch (IOException e) {
             JOptionPane.showMessageDialog(null, "Verbindung zum Server \"" + address + "\" fehlgeschlagen.");
         } finally {
+            if (null != connectionToServer) {
                 try {
                     connectionToServer.close();
                 } catch (IOException e) {
                     e.printStackTrace();
                 }
+            }
 
             try {
                 fromServerReader.close();