При переходе в это окно кидает InvocationTargetException и NullPointerException

Question

При переходе в это окно кидает java.lang.reflect.InvocationTargetException и Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.Button.setOnAction(javafx.event.EventHandler)" because "this.backButton" is null

package com.example.myapp;
import java.net.URL;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;
//                                                          ОКНО АВТОРИЗАЦИИ
public class app6Controller {
@FXML
private ResourceBundle resources;

@FXML
private URL location;

@FXML
private Button ExitButton;

@FXML
private TextField LoginField;

@FXML
private PasswordField PasswordField;

@FXML
private Button regButton;

@FXML
private Button signButton;

@FXML
void initialize() {
            signButton.setOnAction(event -> {
        String login = LoginField.getText().trim();
        String password = PasswordField.getText().trim();

        if(login.equals("") && password.equals("")){
            loginUser(login, password);
        } else {
            System.out.println("Error");
        }
    });
}

private void loginUser(String login, String password) {
    DataBaseHandler dataBaseHandler = new DataBaseHandler();
    User user = new User();
    user.setLogin(login);
    user.setPassword(password);
    ResultSet resultSet = dataBaseHandler.getUser(user);
    int count = 0;
    while (true){
        try {
            if (!resultSet.next()) break;
        } catch (SQLException e) {
            e.printStackTrace();
        }
        count++;
    }
    if(count >= 1){
        System.out.println("Success");
    }
}


}

И его fxml

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.Cursor?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.PasswordField?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>
<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="497.0" prefWidth="700.0" style="-fx-background-color: #2E3349; -fx-border-radius: 10;" xmlns="http://javafx.com/javafx/17" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.example.myapp.app3Controller">
    <children>
        <AnchorPane layoutX="-8.0" layoutY="85.0" prefHeight="419.0" prefWidth="708.0" style="-fx-background-color: #fafafa; -fx-border-radius: 5;">
            <children>
                <Button fx:id="signButton" layoutX="558.0" layoutY="363.0" mnemonicParsing="false" prefHeight="45.0" prefWidth="137.0" style="-fx-background-color: #ffcc66; -fx-background-radius: 0;" text="Войти">
                    <font>
                        <Font name="ft40 Roman" size="16.0" />
                    </font>
                </Button>
                <Label layoutX="14.0" layoutY="363.0" prefHeight="45.0" prefWidth="317.0" text="Авторизация">
                    <font>
                        <Font name="ft40 Roman" size="44.0" />
                    </font>
                </Label>
                <Label layoutX="23.0" layoutY="349.0" opacity="0.13" prefHeight="45.0" prefWidth="317.0" text="Авторизация">
                    <font>
                        <Font name="ft40 Roman" size="44.0" />
                    </font>
                </Label>
                <Label layoutX="175.0" layoutY="14.0" prefHeight="77.0" prefWidth="374.0" text="Введите ваш ник:">
                    <font>
                        <Font name="ft40 Roman" size="40.0" />
                    </font>
                </Label>
                <TextField fx:id="LoginField" layoutX="216.0" layoutY="98.0" prefHeight="37.0" prefWidth="277.0" promptText="Fareny">
                    <font>
                        <Font name="ft40 Roman" size="20.0" />
                    </font>
                </TextField>
                <PasswordField fx:id="PasswordField" layoutX="216.0" layoutY="222.0" prefHeight="37.0" prefWidth="277.0" promptText="123456789">
                    <font>
                        <Font name="ft40 Roman" size="15.0" />
                    </font>
                </PasswordField>
                <Label layoutX="185.0" layoutY="146.0" prefHeight="57.0" prefWidth="354.0" text="Введите пароль:">
                    <font>
                        <Font name="ft40 Roman" size="40.0" />
                    </font>
                </Label>
                <Button fx:id="regButton" layoutX="358.0" layoutY="363.0" mnemonicParsing="false" prefHeight="45.0" prefWidth="200.0" style="-fx-background-radius: 0; -fx-background-color: #2E3349;" text="Зарегистрироваться" textFill="WHITE">
                    <font>
                        <Font name="ft40 Roman" size="16.0" />
                    </font>
                </Button>
            </children>
        </AnchorPane>
        <Label layoutX="14.0" layoutY="10.0" prefHeight="45.0" prefWidth="189.0" text="MineHelper" textFill="WHITE">
            <font>
                <Font name="ft40 Roman" size="30.0" />
            </font>
        </Label>
        <Button fx:id="ExitButton" layoutX="558.0" layoutY="15.0" mnemonicParsing="false" prefHeight="37.0" prefWidth="128.0" style="-fx-background-color: #ffcc66;" text="Exit">
            <font>
                <Font name="ft40 Roman" size="18.0" />
            </font>
        </Button>
    </children>
    <cursor>
        <Cursor fx:constant="DEFAULT" />
    </cursor>
</AnchorPane>