Javafx indexOutOfBoundsException within Textfield due to caret Postion

Question

I'm building a small app where TextField text is updated or deleted via buttons on screen. I have utilised code from this post to be able to move the caret around and add text. I can add and delete characters within the TextField however when n amount of characters are added to the TextField then n amount are removed an IndexOutOfBoundsException occurs when you attempt to add another character as the caret position (when all characters are removed) becomes 1 when it should be 0. I do not know why it becomes 1 when it should be 0. What have I done wrong here?

import java.util.concurrent.atomic.AtomicInteger;
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) {
        try {
            
            BorderPane root = new BorderPane();
            Pane panel = new Pane();
            VBox box = new VBox();
            TextField tf = new TextField();
            Button characterButton = new Button("a");
            Button deleteChar = new Button("delete");
            
            box.getChildren().addAll(tf, characterButton, deleteChar );
            panel.getChildren().add(box);
            root.getChildren().add(panel);
            
            Scene scene = new Scene(root,400,400);
            scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
            primaryStage.setScene(scene);
            primaryStage.show();
            
            //handle caret pos
            AtomicInteger caretPos = new AtomicInteger();
            tf.caretPositionProperty().addListener((obs, oldVal, newVal) -> {
                System.out.println("newval" + newVal);
                if (tf.isFocused()) {
                    caretPos.set(newVal.intValue());
                    System.out.println("Innernewval" + newVal);
                }
            });
            //add character
            characterButton.setOnAction(( event) -> {
                Button btn = (Button)event.getSource();
                System.out.println("Pressed " + btn.getText() + " button");
                int pos = caretPos.get();
                tf.insertText(pos, btn.getText());
                tf.requestFocus();
                tf.positionCaret(pos+1);
                System.out.println("caretPos on add: " + caretPos.get());

            });
            //remove character
            deleteChar.setOnAction(( event) -> {
                Button btn = (Button)event.getSource();
                System.out.println("Pressed " + btn.getText() + " button");
                int newCaretPos = caretPos.get()-1;
                tf.deleteText(newCaretPos, caretPos.get());
                tf.requestFocus();
                tf.positionCaret(newCaretPos);   
                System.out.println("caretPos on delete: " + caretPos.get());
            });
            
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
    
    public static void main(String[] args) {
        launch(args);
    }
}