如何在打开新窗口时修复“ java.lang.NullPointerException”？

Question

我有一些JavaFX程序，我需要在那里打开一个新窗口。我之前使用过此解决方案，并且效果很好。但是，当我再次使用它时，则导致了NullPointerException。我该怎么办？

在此阶段，我尝试将“ txtload” FX ID（第89行）更改为任意一个元素。

java代码

@FXML
    private Button signinbutton;

@FXML
    private Label txtload;

//Problematic code
try {
   Stage oldstage = (Stage) txtload.getScene().getWindow();
   oldstage.close();

   FXMLLoader fxmlLoader = new 
   FXMLLoader(getClass().getResource("../FXML/Main.fxml"));
   Parent root = fxmlLoader.load();
   Stage stage = new Stage();
   stage.setTitle("DenLabs");
   stage.setScene(new Scene(root));
   stage.show();
} catch (IOException e) {
   e.printStackTrace();
}

//Code that works well
try {
   Stage oldstage = (Stage) signinbutton.getScene().getWindow();
   oldstage.close();

   FXMLLoader fxmlLoader = new 
   FXMLLoader(getClass().getResource("../FXML/StartLoading.fxml"));
   Parent root1 = fxmlLoader.load();
   Stage stage = new Stage();
   stage.setTitle("DenLabs - Loading");
   stage.setScene(new Scene(root1));
   stage.show();
   SignInUser(loginText, passwordText);
} catch (IOException e) {
   e.printStackTrace();
}

FXML代码

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

<?import javafx.geometry.Insets?>
<?import javafx.geometry.Rectangle2D?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ProgressBar?>
<?import javafx.scene.image.Image?>
<?import javafx.scene.image.ImageView?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Font?>

<VBox prefHeight="720.0" prefWidth="1280.0" style="-fx-background-color: linear-gradient(#e91e63 0%, #ad1457 100%);" xmlns="http://javafx.com/javafx/8.0.172-ea" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.denlabs.Controllers.SignInController">
   <children>
      <ImageView fitHeight="120.0" fitWidth="120.0" pickOnBounds="true" preserveRatio="true">
         <image>
            <Image url="@../Images/logo_white.png" />
         </image>
         <viewport>
            <Rectangle2D />
         </viewport>
         <VBox.margin>
            <Insets left="580.0" />
         </VBox.margin>
      </ImageView>
      <Label fx:id="txtload" alignment="CENTER" contentDisplay="CENTER" prefWidth="1280.0" text="Loading..." textFill="WHITE">
         <font>
            <Font name="Righteous" size="36.0" />
         </font>
         <VBox.margin>
            <Insets top="40.0" />
         </VBox.margin>
      </Label>
      <ProgressBar prefHeight="10.0" prefWidth="400.0" progress="1.0" style="-fx-text-box-border: none; -fx-accent: #fff;" visible="false">
         <VBox.margin>
            <Insets left="440.0" top="40.0" />
         </VBox.margin>
      </ProgressBar>
   </children>
   <padding>
      <Insets top="265.0" />
   </padding>
</VBox>

我要打开的FXML文件为空（仅AnchorPane）

执行有问题的代码时出错：

java.lang.NullPointerException
    at com.denlabs.Controllers.SignInController.SignInUser(SignInController.java:89)
    at com.denlabs.Controllers.SignInController.SignIn(SignInController.java:54)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Node.fireEvent(Node.java:8411)
    at javafx.scene.control.Button.fire(Button.java:185)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:394)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$358(GlassViewEventHandler.java:432)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:431)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$152(WinApplication.java:177)
    at java.lang.Thread.run(Thread.java:748)

谢谢。