fixed Problem setting Username in ClientConnectionHandler

#42

client/src/main/java/ch/zhaw/pm2/multichat/client/ClientConnectionHandler.java

@@ -39,7 +39,7 @@ public class ClientConnectionHandler extends ConnectionHandler implements Runnab
         this.serverAddress.set(serverAddress);
         this.serverPort.set(serverPort);
         setConnection(NetworkHandler.openConnection(serverAddress, serverPort));
-        this.userName = new SimpleStringProperty((userName == null || userName.isBlank())? USER_NONE : userName);
+        this.userName.set((userName == null || userName.isBlank())? USER_NONE : userName);
     }
 
     public SimpleStringProperty getServerAddressProperty() { return serverAddress; }