我正在嘗試加載FXML文件并將其顯示為應用程序窗口，但出現異常。FXML文件是由FXML Scene Builder創建的。這是班級的代碼public class Main extends Application {    public static void main(String[] args) {        launch(args);    }    @Override    public void start(Stage primaryStage) throws Exception {        primaryStage.setScene(FXMLLoader.load(getClass().getResource("sample.fxml")));        primaryStage.show();    }}和FXML文件<?xml version="1.0" encoding="UTF-8"?><?import javafx.scene.control.Button?><?import javafx.scene.control.TitledPane?><?import javafx.scene.layout.AnchorPane?><TitledPane animated="false" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity"            prefHeight="400.0" prefWidth="600.0" text="Pass4D" xmlns:fx="http://javafx.com/fxml/1"            xmlns="http://javafx.com/javafx/8">    <content>        <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="180.0" prefWidth="200.0">            <children>                <Button layoutX="211.0" layoutY="134.0" mnemonicParsing="false" prefHeight="33.0" prefWidth="177.0"                        text="Log in"/>                <Button layoutX="212.0" layoutY="170.0" mnemonicParsing="false" prefHeight="33.0" prefWidth="175.0"                        text="Exit"/>            </children>        </AnchorPane>    </content></TitledPane>這是我得到的例外Exception in Application start methodException in thread "main" java.lang.RuntimeException: Exception in Application start method    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:875)    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$147(LauncherImpl.java:157)    at com.sun.javafx.application.LauncherImpl$$Lambda$1/2074407503.run(Unknown Source)    at java.lang.Thread.run(Thread.java:745)Caused by: java.lang.NullPointerException: Location is required.    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3201)我究竟做錯了什么？