NullException when binding a property of a nested controller

I want to get the value of a textfield that is changed in one fxml interface (nested.fxml) in another view (main.fxml). I have defined a nestedController like this:

`   
package reproduced;
import java.net.URL;
import java.util.ResourceBundle;

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.TextField;

public class NestedController implements Initializable{
    @FXML
    private TextField changeValue;

    public ObjectProperty<ObjectStringToUpdate> attVal= new SimpleObjectProperty<>();

    @Override
    @FXML
    public void initialize(URL location, ResourceBundle resources) {
        this.changeValue.textProperty().addListener(new ChangeListener<Object>() {
	 
        public void changed(ObservableValue<?> ov, Object t, Object t1) {
            ObjectStringToUpdate newValue = new ObjectStringToUpdate();  
            newValue.setAttValNested(t1.toString());
            attVal.setValue(newValue);
		}
 
});
}

}`

And the mainController is:

`import java.io.IOException;
import java.net.URL;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.Property;
import javafx.beans.property.SimpleObjectProperty;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;

public class MainController implements Initializable{
    @FXML
    public BorderPane mainBorderPane;
    Property<ObjectStringToUpdate> attVal =  new SimpleObjectProperty<>();
	 
    @FXML public NestedController nestedController;//injected nested controller
    @Override
    public void initialize(URL location, ResourceBundle resources) {
        ObjectProperty<ObjectStringToUpdate> binding = nestedController.attVal;
        ((Property<ObjectStringToUpdate>) this.attVal).bind(binding);
 
        }
    @FXML
   public void testVal() {
        System.out.println(this.attVal.getValue().attValNested);// this when called returns null value because this.attaVal is null
        /*Caused by: java.lang.NullPointerException: Cannot read field "attValNested" because the return value of "javafx.beans.property.Property.getValue()" is null
*/
    }
    @FXML 
    public void openNestedView() throws IOException {
    	FXMLLoader loaderOfTabs = new 
    	        FXMLLoader(getClass().getResource("/nested.fxml"));
    	AnchorPane tabPane = (AnchorPane) loaderOfTabs.load();
    	mainBorderPane.setCenter(tabPane);
        }
 
}`

Then the object:

`public class ObjectStringToUpdate {
    public String attValNested;
    
    public String getAttValNested() {
        return attValNested;
    }
    
    public void setAttValNested(String attVal) {
    	this.attValNested = attVal;
        }	
    }`

The app main entry is:

`package reproduced;

import java.io.IOException;

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

public class UInterface2 extends Application{
    @Override
    public void start(Stage stage) throws IOException {
        String fxmlFile = "main.fxml";
        FXMLLoader loader = new FXMLLoader();
        Parent rootNode = (Parent) loader.load(getClass().getClassLoader().getResource(fxmlFile));
        Scene scene = new Scene(rootNode, 1024, 843);
        stage.setTitle("reproduced");
        stage.setResizable(false);
        stage.setScene(scene);
        stage.show();
    }

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

}`

fxmls:

main.fxml:

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.layout.*?>
<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<AnchorPane xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:id="rootAnchorPane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="843.0" prefWidth="1224.0" fx:controller="reproduced.MainController">
   <children>
      <BorderPane fx:id="mainBorderPane" layoutY="-1.0" prefHeight="843.0" prefWidth="1024.0">
         <top>
            <Button mnemonicParsing="false" onAction="#openNestedView" text="open nested View" BorderPane.alignment="CENTER" />
         </top>
      </BorderPane>
      <Button fx:id="testVal" mnemonicParsing="false" onAction="#testVal" text="test" />
      <Label fx:id="showValue" text="Label" />
   </children>
   <fx:include fx:id="nested" source="nested.fxml" visible="false" />
</AnchorPane>`

nested.fxml:

<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<AnchorPane xmlns="http://javafx.com/javafx/17" xmlns:fx="http://javafx.com/fxml/1" fx:id="rootAnchorPaneInner" fx:controller="reproduced.NestedController">
   <children>
      <TextField xmlns="http://javafx.com/javafx/8" fx:id="changeValue" />
   </children>
</AnchorPane>`

I have also included the fxml component of NestedController in main.fxml properly.

But when I update the textfield value in the nested controller and try to call it in the main Controller through a button click I get null exception: the return value of "javafx.beans.property.ObjectProperty.getValue()" is null

I expect the this.attVal at MainController.testVal() to return the value as changed in the NestedController.

You are loading the FXML file nested.fxml twice. Once via the <fx:include> element in main.fxml, and then subsequently each time you press the "open nested view" button.

The instance of nested.fxml loaded via <fx:include> is not actually visible, since you (for some reason) have visible="false" in the <fx:include> element. The instance that appears when the button is pressed is a different instance entirely, created via the call to loaderOfTabs.load().

The nested controller, of course, is associated with the instance created by <fx:include>. Since that is not visible, the user can never type in its text field, and so its text field's textProperty.onChanged handler can never be invoked. Consequently, the value of attVal in that instance of the controller is never initialized: i.e. in that controller instance attVal.getValue() will always be null, and so when you invoke attVal.getValue().attValNested you of course get a NullPointerException.

I don't understand why you have multiple instances of the nested.fxml UI. Either use <fx:include> or load it in code. If you want to use <fx:include> and keep it invisible until the user presses a button (why???), just do:

public class MainController implements Initializable{

    // ...

    @FXML public NestedController nestedController;//injected nested controller
    @FXML private AnchorPane nested;


    // ...

    public void openNestedView() throws IOException {
        nested.setVisible(true);
    }

}

Conversely, if you want to load the FXML when the button is pressed, remove the <fx:include> from main.fxml:

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<AnchorPane xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:id="rootAnchorPane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="843.0" prefWidth="1224.0" fx:controller="org.jamesd.examples.nestedprop.MainController">
    <children>
        <BorderPane fx:id="mainBorderPane" layoutY="-1.0" prefHeight="843.0" prefWidth="1024.0">
            <top>
                <Button mnemonicParsing="false" onAction="#openNestedView" text="open nested View" BorderPane.alignment="CENTER" />
            </top>

        </BorderPane>
        <Button fx:id="testVal" mnemonicParsing="false" onAction="#testVal" text="test" />
        <Label fx:id="showValue" text="Label" />
    </children>
</AnchorPane>

and then just get the controller from the FXMLLoader and perform the binding when you get it:

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

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.Property;
import javafx.beans.property.SimpleObjectProperty;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;

public class MainController implements Initializable{
    @FXML
    public BorderPane mainBorderPane;
    Property<ObjectStringToUpdate> attVal =  new SimpleObjectProperty<>();

    private NestedController nestedController;
    @Override
    public void initialize(URL location, ResourceBundle resources) {

    }
    @FXML
    public void testVal() {
        System.out.println(this.attVal.getValue().attValNested);
    }
    @FXML
    public void openNestedView() throws IOException {
        FXMLLoader loaderOfTabs = new
                FXMLLoader(getClass().getResource("nested.fxml"));
        AnchorPane tabPane = (AnchorPane) loaderOfTabs.load();

        nestedController = loaderOfTabs.getController();

        ObjectProperty<ObjectStringToUpdate> binding = nestedController.attVal;
        this.attVal.bind(binding);

        mainBorderPane.setCenter(tabPane);

    }

}