當(dāng)我按下按鈕 loadMap 時，我應(yīng)該能夠選擇一個文件（圖像）并且它應(yīng)該被上傳到我擁有的 borderpane 的中心，但是一旦我選擇一個文件，我就會不斷收到錯誤和 NullPointerException。我試過將它作為內(nèi)部類作為處理程序，但它是同樣的問題。這是以下代碼：public class Karta extends Application {    final int LIST_CELL_HEIGHT = 26;    TextField textfield;    RadioButton b1;    RadioButton b2;    Label label = new Label("Categories");    ListView<String> lView;    Button hideC;    Button New;    Button search;    Button hide;    Button remove;    Button coordinate;    FileChooser fileChooser;    File file;    ImageView imageView;    ObservableList<String> items = FXCollections.observableArrayList("Bus", "Train", "Underground");    ListView<String> list = new ListView<>(items);    MenuBar fileMenu;    Stage primaryStage;    public static void main(String[] args) {        launch(args);    }    @Override    public void start(Stage primaryStage) {        this.primaryStage = primaryStage;        BorderPane root = new BorderPane();        New = new Button("New");        b1 = new RadioButton("Named");        b2 = new RadioButton("Described");        ToggleGroup group = new ToggleGroup();        b2.setToggleGroup(group);        b1.setToggleGroup(group);        VBox vInTopH = new VBox(5);        vInTopH.getChildren().addAll(b1, b2);        textfield = new TextField("Search..");        search = new Button("Search");        hide = new Button("Hide");        remove = new Button("Remove");        coordinate = new Button("Coordinate");        HBox topHbox = new HBox(5);        topHbox.getChildren().addAll(New, vInTopH, textfield, search, hide, remove, coordinate);        topHbox.setAlignment(Pos.CENTER);        VBox topVbox = new VBox(10);結(jié)果應(yīng)該是所選文件在設(shè)置時填充邊框的中心。