JavaFX: setting list for comboBox does not work

Question

I am working in a JavaFX project. There I have fxml included in other fxml files. Now I need to send data from the outer controller to the inner controller. I've tried it, but the combobox looses its items as soon as the transferMethod finished.

Edit: As in the comment section advised, I switched to another import method. This import method fixes the bug with the comboBox, but causes problems with the import itself, at least in some cases. Therefore I update the MRE

Parts in use:
Import class:

    package at.palberon.miniproqa.fxplayground;

import java.util.ArrayList;

public class Import {

    ArrayList<String> list;

    public Import() {
        list = new ArrayList<>();
        list.add("Hello");
        list.add("World");
    }

    public ArrayList<String> getList() {
        return list;
    }


}

inner.fxml:

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

<?import javafx.scene.control.ComboBox?>
<?import javafx.scene.layout.AnchorPane?>

<AnchorPane  xmlns="http://javafx.com/javafx/22" xmlns:fx="http://javafx.com/fxml/1" fx:controller="at.palberon.miniproqa.fxplayground.InnerController">
    <ComboBox fx:id="comboBox" prefWidth="150.0" />
</AnchorPane>

InnerController Class:

package at.palberon.miniproqa.fxplayground;

import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.fxml.FXML;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;

import java.util.ArrayList;

public class InnerController {

    @FXML
    ComboBox<String> comboBox;

    ObservableList<String> observableList;

    public void setComboBoxItems(ArrayList<String> items) {
        observableList = FXCollections.observableArrayList(items);
        comboBox.setItems(observableList);
    }

}

outer.fxml:

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

<?import javafx.scene.control.Tab?>
<?import javafx.scene.control.TabPane?>


<TabPane prefHeight="600.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/22" xmlns:fx="http://javafx.com/fxml/1"
         fx:controller="at.palberon.miniproqa.fxplayground.OuterController">
    <tabs>
        <Tab fx:id="efTab" closable="false" text="Einstieg">
            <fx:include fx:id="innerController" source="inner.fxml"/>
        </Tab>
    </tabs>
</TabPane>

OuterController Class:

package at.palberon.miniproqa.fxplayground;

import javafx.fxml.FXML;
import java.util.ArrayList;

public class OuterController {

    private ArrayList<String> items;


    @FXML
    private InnerController innerController;

    public void initialize(){

        if (innerController != null) {
            System.out.println("Right");
        }

        Import importData = new Import();
        items = importData.getList();

        innerController.setComboBoxItems(items);
    }
}

With this setup, I am unable to start the program, and get an Exception.

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:118)
    at java.base/java.lang.reflect.Method.invoke(Method.java:580)
    at javafx.graphics@22-ea/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
    at javafx.graphics@22-ea/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:364)
    at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
    at java.base/java.lang.reflect.Method.invoke(Method.java:580)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1147)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics@22-ea/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:893)
    at javafx.graphics@22-ea/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:196)
    at java.base/java.lang.Thread.run(Thread.java:1570)
Caused by: javafx.fxml.LoadException: 
/C:/Users/Pascal/IdeaProjects/fxplayground/target/classes/at/palberon/miniproqa/fxplayground/outer.fxml:11

    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2727)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2705)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2568)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2536)
    at at.palberon.miniproqa.fxplayground/at.palberon.miniproqa.fxplayground.HelloApplication.start(HelloApplication.java:14)
    at javafx.graphics@22-ea/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:839)
    at javafx.graphics@22-ea/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:483)
    at javafx.graphics@22-ea/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:456)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:400)
    at javafx.graphics@22-ea/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:455)
    at javafx.graphics@22-ea/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at javafx.graphics@22-ea/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics@22-ea/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:185)
    ... 1 more
Caused by: java.lang.IllegalArgumentException: Can not set at.palberon.miniproqa.fxplayground.InnerController field at.palberon.miniproqa.fxplayground.OuterController.innerController to javafx.scene.layout.AnchorPane
    at java.base/jdk.internal.reflect.FieldAccessorImpl.throwSetIllegalArgumentException(FieldAccessorImpl.java:228)
    at java.base/jdk.internal.reflect.FieldAccessorImpl.throwSetIllegalArgumentException(FieldAccessorImpl.java:232)
    at java.base/jdk.internal.reflect.MethodHandleObjectFieldAccessorImpl.set(MethodHandleObjectFieldAccessorImpl.java:115)
    at java.base/java.lang.reflect.Field.set(Field.java:836)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.injectFields(FXMLLoader.java:1180)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader$ValueElement.processValue(FXMLLoader.java:870)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:764)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2858)
    at javafx.fxml@22-ea/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2654)
    ... 12 more
Exception running application at.palberon.miniproqa.fxplayground.HelloApplication

Process finished with exit code 1

I have an inner module, I call it a module (fxml file plus the Controller) which is included in the outer module. In the FXML of the outer module, I include the FXML of the inner. In the outer Controller I try to make me the inner Controller accessible.