The controller class (ClientChatController.java) fxml being used (clientGUI.fxml) and the JFXDialog that needs to be used (dialog.fxml)

ClientChatController.java

package client.gui;

import com.jfoenix.controls.*;
import javafx.collections.FXCollections;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.scene.text.Text;

import java.net.URL;
import java.util.ArrayList;
import java.util.ResourceBundle;

/**
 * @author Umar
 *         Class used to respond to events from clientGUI.fxml
 */
public class ClientChatController implements Initializable
{
	private ArrayList<Label> users = new ArrayList<Label>();
	@FXML
	private JFXListView<Label> userList;
	@FXML
	private JFXListView<Label> conversationList;
	@FXML
	private StackPane stackPane;
	@FXML
	private JFXTextField username;
	@FXML
	private JFXTextField firstname;
	@FXML
	private JFXTextField lastname;
	@FXML
	private JFXTextField serverIP;

	@Override
	public void initialize(URL location, ResourceBundle resources)
	{

		//open dialog that contains text field for username, firstname,
		//lastname and server IP address

		JFXDialogLayout content = new JFXDialogLayout();
		content.setHeading(new Text("Setup"));
		VBox vbox = new VBox(10);
		vbox.getChildren()
				.addAll(username, firstname, lastname, serverIP);
		content.setBody(vbox);
		JFXDialog dialog = new JFXDialog(stackPane, content, JFXDialog.DialogTransition.CENTER);
		JFXButton button = new JFXButton("Connect");
		button.setOnAction(new EventHandler<ActionEvent>()
		{
			@Override
			public void handle(ActionEvent event)
			{
				System.out.println("Username: " + username.getText());
				System.out.println("Firstname: " + firstname.getText());
				System.out.println("Lastname: " + lastname.getText());
				System.out.println("ServerIP: " + serverIP.getText());
				dialog.close();
			}
		});

		content.setActions(button);
		dialog.show();

	}

	@FXML
	public void clientSetup(ActionEvent event)
	{
		System.out.println("Username: " + username);
		System.out.println("Firstname: " + firstname);
		System.out.println("Lastname: " + lastname);
		System.out.println("ServerIP: " + serverIP);
	}
}

clientGUI.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import com.jfoenix.controls.*?>
<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ScrollPane?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.text.Text?>
<StackPane fx:id="stackPane"
           xmlns="http://javafx.com/javafx/8.0.112-ea"
           xmlns:fx="http://javafx.com/fxml/1"
           fx:controller="client.gui.ClientChatController">
	<BorderPane
	>
		<left>
			<ScrollPane fitToHeight="true"
			            fitToWidth="true">
				<content>
					<JFXListView fx:id="userList"/>
				</content>
			</ScrollPane>
		</left>
		<right>
			<BorderPane>
				<center>
					<ScrollPane fitToWidth="true">
						<content>
							<JFXListView fx:id="conversationList"/>
						</content>
					</ScrollPane>
				</center>
				<bottom>
					<BorderPane>
						<padding>
							<Insets bottom="5"
							        top="1"/>
						</padding>
						<center>
							<JFXTextArea fx:id="textArea"
							             maxHeight="92"
							             maxWidth="450"
							             promptText="Type message"
							             disable="true">
							</JFXTextArea>
						</center>
						<right>
							<JFXButton fx:id="sendBtn"
							           alignment="CENTER"
							           buttonType="FLAT"
							           disable="true"
							           maxHeight="1.7976931348623157E308"
							           maxWidth="110.0"
							           ripplerFill="#4059a9"
							           text="Send"
							           textAlignment="CENTER">
							</JFXButton>
						</right>
					</BorderPane>
				</bottom>
			</BorderPane>
		</right>
	</BorderPane>
	
	<JFXDialog fx:id="setupDialog">
		<JFXDialogLayout>
			<heading>
				<Label>Setup</Label>
			</heading>
			<body>
				<VBox spacing="10">
					<JFXTextField fx:id="username"
					              promptText="Username"
					              >Username
					</JFXTextField>
					<JFXTextField fx:id="firstname"
					              promptText="First name"
					              >Firstname
					</JFXTextField>
					<JFXTextField fx:id="lastname"
					              promptText="Last name"
					              >Lastname
					
					</JFXTextField>
					<JFXTextField fx:id="serverIP"
					              promptText="Server IP address"
					              >Server IP Address
					</JFXTextField>
				</VBox>
			</body>
			<actions>
				<HBox>
					<Text fx:id="errorMessage"></Text>
					<JFXButton fx:id="connectBtn" onAction="#clientSetup">Connect</JFXButton>
				</HBox>
			</actions>
		</JFXDialogLayout>
	</JFXDialog>
</StackPane>

dialog.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import com.jfoenix.controls.*?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.StackPane?>
<?import javafx.scene.layout.VBox?>
<StackPane
		xmlns:fx="http://javafx.com/fxml/1"
		xmlns="http://javafx.com/javafx/2.2"
		fx:controller="client.gui.ClientChatController">
	<JFXDialog fx:id="setupDialog">
		<JFXDialogLayout>
			<heading>
				<Label>Setup</Label>
			</heading>
			<body>
				<VBox spacing="10">
					<JFXTextField fx:id="username"
					              promptText="Username"
					              maxWidth="20">
					
					</JFXTextField>
					<JFXTextField fx:id="firstName"
					              promptText="First name"
					              maxWidth="20">
					
					</JFXTextField>
					<JFXTextField fx:id="lastname"
					              promptText="Last name"
					              maxWidth="20">
					
					</JFXTextField>
					<JFXTextField fx:id="serverIP"
					              promptText="Server IP address"
					              maxWidth="20">
					
					</JFXTextField>
				</VBox>
			</body>
			<actions>
				<JFXButton fx:id="connectBtn">Connect</JFXButton>
			</actions>
		</JFXDialogLayout>
	</JFXDialog>
</StackPane>