Javafx场景构建器CSS SetStyle覆盖

Question

我正在开发基本的javafx GUI，我需要创建更改背景颜色的按钮。我希望“Nightmode”按钮改变背景颜色，但它会导致“java.lang.NullPointerException”我知道，这个错误意味着什么，但我不知道为什么以及它来自何处。有什么建议吗？

这是我的代码：

Controler.java

package sample;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.TextArea;
import javafx.scene.control.TextField;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.layout.AnchorPane;

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

public class Controller implements Initializable {


@FXML
private TextField primesAmount;
@FXML
private TextField noArgument;
@FXML
private TextArea showPrime;
@FXML
private Primes primes = new Primes();
@FXML
private AnchorPane background;


@FXML
private void handleAction(ActionEvent event) {
    try {
        primes.createTabel(Integer.parseInt(primesAmount.getText()));
    } catch (NumberFormatException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText("NOT AN INTEGEER");
        alert.setContentText("Try again!");
        alert.showAndWait();

    } catch (IllegalArgumentException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText(" illegal or inappropriate argument!");
        alert.setContentText("Try again!");
        alert.showAndWait();

    } catch (NegativeArraySizeException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText(" Out of range");
        alert.setContentText("Try again!");
        alert.showAndWait();

    }
    }


@FXML
private void handleAction2(ActionEvent event) {
    try {
        showPrime.setText(Integer.toString(primes.number(Integer.parseInt(noArgument.getText()))));
    } catch (NumberFormatException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText("NOT AN INTEGEER");
        alert.setContentText("Try again!");
        alert.showAndWait();

    } catch (IllegalArgumentException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText(" illegal or inappropriate argument!");
        alert.setContentText("Try again!");
        alert.showAndWait();

    } catch (NegativeArraySizeException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText(" Out of range");
        alert.setContentText("Try again!");
        alert.showAndWait();

    }
    catch (ArrayIndexOutOfBoundsException ex) {
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("ERROR");
        alert.setHeaderText(" Out of range");
        alert.setContentText("Try again!");
        alert.showAndWait();

    }

}

@FXML
private void Exit(ActionEvent event) {
    System.exit(1);

}

@FXML
private void Nightmode(ActionEvent event) {
    background.setStyle("-fx-background-color: grey");

}

@Override
public void initialize(URL url, ResourceBundle rb) {
}
}

主。的java：

package sample;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

import javax.swing.text.html.StyleSheet;

public class Main extends Application{

@Override
public void start(Stage primaryStage) throws Exception{
    Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
    primaryStage.setTitle("Prime counter");
    primaryStage.setScene(new Scene(root));

    primaryStage.show();
}


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


}

CSS：

#button1 {
 -fx-background-color: gold;
}

#button2 {
-fx-background-color: #4CAF50; /* Green */
-fx-border: none;
-fx-color: white;

-fx-text-align: center;
-fx-text-decoration: none;
-fx-display: inline-block;
-fx-font-size: 16px;
}



#background {
-fx-background-color: cornflowerblue;
}

sample.fxml：

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.TextArea?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>

<AnchorPane id="background" maxHeight="-Infinity" maxWidth="-Infinity"minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8.0.121" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
<children>
    <TextField fx:id="primesAmount" alignment="CENTER" layoutX="35.0" layoutY="90.0" text="Enter no. primes here" />
    <Button id="button1" layoutX="215.0" layoutY="90.0" mnemonicParsing="false" onAction="#handleAction" stylesheets="@StyleSheet.css" text="O.K.!" />
    <TextField fx:id="noArgument" layoutX="35.0" layoutY="134.0" />
    <Button id="button2" fx:id="confirm" layoutX="215.0" layoutY="134.0" mnemonicParsing="false" onAction="#handleAction2" stylesheets="@StyleSheet.css" text="Confirm" />
    <TextArea fx:id="showPrime" disable="true" editable="false" layoutX="225.0" layoutY="207.0" opacity="0.29" prefHeight="130.0" prefWidth="151.0">
        <font>
            <Font size="50.0" />
        </font>
    </TextArea>
    <Button layoutX="525.0" layoutY="337.0" mnemonicParsing="false" onAction="#Exit" text="Exit" />
    <Button layoutX="500.0" layoutY="38.0" mnemonicParsing="false" onAction="#Nightmode" text="Night mode" />
</children>
</AnchorPane>