JavaFX server client program

I am making a program, and I don't know why my program becomes unresponsive, when I click the button ):
I have a feeling its because of this:
"connection = server.accept();"
I don't know how to fix it.

I am trying to recreate this:
https://stackoverflow.com/questions/18505123/how-to-use-client-chat-on-different-pc

but in javafx.

package yeetYeet;

import java.io.EOFException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextArea;
import javafx.scene.control.TextField;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Server extends Application{

       private ObjectOutputStream output;
       private ObjectInputStream input;
       private ServerSocket server;
       private Socket connection;

    //set up and run the server
    private void startRunning() {
        try{
             server = new ServerSocket(6789, 100);

             new AnimationTimer() {

                @Override
                public void handle(long arg0) {
                    try{
                       waitForConnection();
                       setupStreams();
                       whileChatting();
                    }catch(EOFException eofException){
                       showMessage("\n Server ended the connection! ");
                    }catch (IOException IOException){
                        IOException.printStackTrace();
                    }finally{
                       closeCrap();
                    }
                }
             }.start();
          }catch(IOException ioException){
             ioException.printStackTrace();
          }
    }

    //wait for connection, then display connection information
    private void waitForConnection() throws IOException {
        showMessage(" Waiting for someone to connect... \n");
        connection = server.accept();
        showMessage(" Now connected to " + connection.getInetAddress().getHostName());
    }

    //get stream to send and receive data
    private void setupStreams() throws IOException {
        output = new ObjectOutputStream(connection.getOutputStream());
        output.flush();
        input = new ObjectInputStream(connection.getInputStream());
        showMessage("\n Streams are now setup! \n");
    }

    //during the chat conversation
    private void whileChatting() throws IOException {
        String message = " You are now connected! ";
        sendMessage(message);
        ableToType(true);
        do {
            try {
                message = (String) input.readObject();
                showMessage("\n" + message);
            }catch(ClassNotFoundException classNotFoundException) {
                showMessage("\n idk wtf that user sent!");
            }
        }while(!message.equals("CLIENT - END"));
    }

    //close streams and sockets after you are done chatting
    private void closeCrap() {
        showMessage("\n Closing connections... \n");
        ableToType(false);
        try {
            output.close();
            input.close();
            connection.close();
        }catch(IOException ioException) {
            ioException.printStackTrace();
        }
    }

    //send a message to client
    private void sendMessage(String message) {
        try {
            output.writeObject("SERVER - " + message);
            output.flush();
            showMessage("\nSERVER - " + message);
        }catch(IOException ioException) {
            ta.appendText("\n ERROR: DUDE I CANT SEND THAT MESSAGE");
        }
    }

    //updates chatWindow
    private void showMessage(final String text) {
        new AnimationTimer() {

            @Override
            public void handle(long arg0) {
                ta.appendText(text);
            }
        }.start();
    }

    //let the user type stuff into their box
    private void ableToType(final boolean tof) {
        new AnimationTimer() {

            @Override
            public void handle(long arg0) {
                tf.setEditable(tof);
            }
        }.start();
    }

    private Group serverGR;
    private Scene serverSC;
    private Stage window;
    private TextField tf;
    private TextArea ta;
    private Button connectingStart;

    @Override
    public void start(Stage primaryStage) throws Exception {
        serverGR = new Group();
        serverSC = new Scene(serverGR, 1280, 720, Color.PALEGREEN);

        window = primaryStage;
        window.setScene(serverSC);
        window.setTitle("Bleidorb -\\\\\"Server\"\\\\- ChatProgram");
        window.show();

        tf = new TextField();
        tf.setTranslateX(10);
        tf.setTranslateY(10);
        tf.setPrefWidth(500);
        tf.setPrefHeight(20);
        tf.appendText("type your message here");
        serverGR.getChildren().add(tf);

        ta = new TextArea();
        ta.setTranslateX(10);
        ta.setTranslateY(40);
        ta.setPrefWidth(500);
        ta.setPrefHeight(500);
        serverGR.getChildren().add(ta);

        connectingStart = new Button("Connection Start");
        connectingStart.setTranslateX(600);
        connectingStart.setTranslateY(300);
        connectingStart.setPrefWidth(150);
        connectingStart.setPrefHeight(30);
        connectingStart.setOnAction(e -> {
            startRunning();
        });
        serverGR.getChildren().add(connectingStart);
    }

    public static void main(String[] args) {
        launch(args);
    }
}