Why my ToggleGroup variable is always null? - javafx

Question

I defined the following FirstWindow.fxml file:
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>
<AnchorPane id="switcherContainer" layoutX="0.0" layoutY="0.0" prefHeight="170.0" prefWidth="200.0"
xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1"
fx:controller="rep.ButtonController">
<fx:define>
<ToggleGroup fx:id="rType"/>
</fx:define>
<Label layoutX="35.0" layoutY="25.0" prefHeight="17.0" prefWidth="260.0" text="Choose the report type:" />
<RadioButton layoutX="35.0" layoutY="60.0" mnemonicParsing="false" text="A" toggleGroup="$rType" />
<RadioButton layoutX="35.0" layoutY="80.0" mnemonicParsing="false" text="B" toggleGroup="$rType" />
<RadioButton layoutX="35.0" layoutY="100.0" mnemonicParsing="false" text="C" toggleGroup="$rType" />
<RadioButton layoutX="35.0" layoutY="120.0" mnemonicParsing="false" text="D" toggleGroup="$rType" />
</AnchorPane>
I added this FirstWindow.fxml file in Main class:
AnchorPane rootPane = (AnchorPane) FXMLLoader.load(getClass().getResource("/rep/RootScene.fxml"));
primaryStage.setTitle("Report Generation");
primaryStage.setScene(new Scene(rootPane));
AnchorPane innerPane = (AnchorPane) FXMLLoader.load(getClass().getResource("/rep/FirstWindow.fxml"));
rootPane.getChildren().addAll(innerPane);
primaryStage.show();
And my ButtonController class looks like this:
public class ButtonController implements Initializable {
static int step = 0;
#FXML
private Button nextButton;
#FXML
ToggleGroup rType;
#Override
public void initialize(URL location, ResourceBundle resources) {
}
public void onNextButtonClick(ActionEvent event) {
System.out.println("Button Clicked!");
try {
if (rType != null) {
System.out.println("RadioButton selected: " + rType.getSelectedToggle().getUserData().toString());
} else {
System.out.println("rType is null");
}
...
primaryStage.show();
} catch (Exception ex) {
System.out.println("Exception: " + ex);
}
}
}
But I always get the following output in the console:
rType is null
Why? And how to fix it?