Close server at the end

src/main/java/ChatClient.java

@@ -60,6 +60,12 @@ 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();
+            }
         }
     }