Cannot add elemento cause it's not a JAVAFX thread

i'm tryng to modify le list every time client send somethig.It have to work like a server log. I reserched for javafx concurrency and i found class Service and Task that are very usefully but when i try to modify the list on the second task, show an error of wrong thread "it's not on javafx thread". There is a way to resolve that? I'm studying now the concurrency and i can't understand the problem.Thanks.

Server: (I call a Connection conn on initialization of application with conn.start())

   public class Connection extends Service<Void>{
      Socket incoming;
      ServerSocket s;
      PrintWriter out;
      @Override
      protected Task<Void> createTask() {
         return new Task<>() {
         @Override
         protected Void call() {
            System.out.println("Server Attivo");
               try {
                  s = new ServerSocket(8189);
                  while (!isCancelled()) {
                     System.out.println("In attesa di connessione....");
                     incoming = s.accept();
                     System.out.println("after accept "+Thread.currentThread());
                     server = new Task<Void>() {
                        @Override
                        protected Void call() throws Exception {
                           System.out.println("Connessione accettata!");
                           System.out.println(Thread.currentThread());
                           InputStream inStream = incoming.getInputStream();
                           OutputStream outStream = incoming.getOutputStream();
                           out = new PrintWriter(outStream, true);
                           Scanner in = new Scanner(inStream);String line;
                           out.println("Connessione accettata!");
                           while (!isCancelled()){
                              line = in.nextLine();
                              System.out.println(line);
                              list.add(new Log(line));  --> the error is here
                           }
                              return null;
                        }
                     };
                     new Thread(server).start();
                  }
               }catch (SocketException ex){
                  System.out.println("Listener chiuso!");
               }
               catch (IOException e) {
                  e.printStackTrace();
               }
            return null;
         }

            @Override
            protected void cancelled() {
               System.out.println("Closing server....");
               System.out.println("Done!");
               if(out!=null)out.println("Connessione chiusa!");
                  try {
                     s.close();
                     try {
                        incoming.close();
                     }catch (NullPointerException EX){
                        System.out.println("Non è stata stabilita nessuna connesione client prima della chiusura!");
                     }
                  }
                  catch (NullPointerException EX){
                     System.out.println("Server non inizializzato correttamente");
                  }
                  catch (IOException e) {
                     e.printStackTrace();
                  }

               if(server!=null)server.cancel();
               super.cancelled();
            }

         };
      }
   }
}