JavaFX TextArea appendText可在初始化中运行，但不能在其他位置运行

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.TextArea;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) {
        TextArea textArea = new TextArea();
        textArea.appendText("Hello, World!");

        VBox root = new VBox(textArea);
        Scene scene = new Scene(root, 400, 300);

        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextArea;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Main extends Application {

    private TextArea textArea;

    @Override
    public void start(Stage primaryStage) {
        textArea = new TextArea();

        Button button = new Button("Append Text");
        button.setOnAction(event -> {
            appendText("New Text");
        });

        VBox root = new VBox(textArea, button);
        Scene scene = new Scene(root, 400, 300);

        primaryStage.setScene(scene);
        primaryStage.show();
    }

    private void appendText(String text) {
        Platform.runLater(() -> textArea.appendText(text));
    }

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