Is it possible to bind the size of an accordion dynamically to the size of the expanded titled pane in JavaFX-8? - size

Question

Such as the title suggests, i'm trying to bind the size of an Accordion dynamically to the size of the expanded TitledPane.
I tried to add an ChangeListener to the expandedPaneProperty of the accordion in the initialize method of the controller for the UI which contains the Accordion using following code:
private double prefHeight, prefWidth;
#Override
public void initialize(URL location, ResourceBundle resources) {
prefHeight = acc.getPrefHeight(); //double variables of controller
prefWidth = acc.getPrefWidth();
acc.expandedPaneProperty().addListener(new ChangeListener<Object>(){
#Override
public void changed(ObservableValue<? extends Object> observable, Object oldValue, Object newValue) {
TitledPane pane = acc.getExpandedPane();
if(pane != null){
acc.setPrefHeight(prefHeight + pane.getPrefHeight());
acc.setPrefWidth(prefWidth + pane.getPrefWidth());
}
}
});
}
The Problem is, that this code leads to a NullPointerException at line 3, which might indicate that the property might not be instatiated at this moment.
Update: Problem concerning NullPointerException was solved, i didn't initialized the Accordion properly.
The code is still not working, though.
The corresponding FXML:
<Accordion fx:id="acc" xmlns="http://javafx.com/javafx/8.0.60" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.SettingsController">
<panes>
<TitledPane alignment="TOP_LEFT" animated="false" text="Alarm sound">
<tooltip>
<Tooltip text="Enables to choose an alarm sound." />
</tooltip>
<content>
<VBox alignment="TOP_CENTER" maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" styleClass="box">
<children>
<HBox alignment="TOP_CENTER" styleClass="hbox">
<children>
<Button mnemonicParsing="false" text="Reset to default" />
<Button mnemonicParsing="false" onAction="#saveAndExitOnClick" text="Save and exit" />
</children>
</HBox>
<ListView maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" VBox.vgrow="ALWAYS" />
</children>
</VBox>
</content></TitledPane>
<TitledPane animated="false" text="Style">
<tooltip>
<Tooltip text="Enables to choose a style for the program." />
</tooltip>
<content>
<VBox maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" styleClass="box">
<children>
<HBox alignment="TOP_CENTER" styleClass="h-box">
<children>
<Button mnemonicParsing="false" text="Reset to default" />
<Button mnemonicParsing="false" onAction="#saveAndExitOnClick" text="Save and exit" />
</children>
</HBox>
<ListView maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" />
</children>
</VBox>
</content></TitledPane>
<TitledPane animated="false" text="Language">
<tooltip>
<Tooltip text="Enables to choose a style for the program." />
</tooltip>
<content>
<VBox maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" styleClass="box">
<children>
<HBox alignment="TOP_CENTER">
<children>
<Button mnemonicParsing="false" text="Reset to default" />
<Button mnemonicParsing="false" onAction="#saveAndExitOnClick" text="Save and exit" />
</children>
</HBox>
<ListView maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" />
</children>
</VBox>
</content>
</TitledPane>
<TitledPane maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" styleClass="box" text="Other settings">
<content>
<ToolBar maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" orientation="VERTICAL" styleClass="box">
<items>
<Button contentDisplay="CENTER" maxWidth="1.7976931348623157E308" mnemonicParsing="false" text="Recenter Window" />
<Button maxWidth="1.7976931348623157E308" mnemonicParsing="false" text="Reset program to default settings" />
<Button maxWidth="1.7976931348623157E308" mnemonicParsing="false" text="Load custom alarm sound" />
<Button maxWidth="1.7976931348623157E308" mnemonicParsing="false" text="Apply custom style" />
</items>
</ToolBar>
</content>
</TitledPane>
<TitledPane maxHeight="1.7976931348623157E308" prefWidth="200.0" text="About">
<content>
<TextArea editable="false" maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" text="Timer Program, written by QBW.
Version:
Fonts:
Font Awesome, License:
Alarm sounds:
Acknowledgement:
·Hans-Peter Habelitz for "Programmieren lernen mit Java"
·Christian Ullenboom for "Java ist auch eine Insel" and "Java SE8 Standardbibliothek"
·Anton Epple for "JavaFX 8 - Grundlagen und fortgeschrittene Techniken"
·The stackoverflow.com community
· Creators of alarm sound files:
·
·
·
·
·
· Creators of Font awesome
" wrapText="true" />
</content>
<tooltip>
<Tooltip text="Contains information about the program, its creator and license information" />
</tooltip>
</TitledPane>
</panes>
</Accordion>
How is it possible to solve that problem?