JavaFx WebView element selection. (JavaScript)

I have a WebView element, which render a html code. I want to create getSelectedIndices and setSelectedIndices method. Getter works, Setter throws IndexSizeException.

How can I set the selection area of WebView?

package hu.krisz;

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.VBox;
import javafx.scene.web.WebView;
import javafx.stage.Stage;

public class WebViewExample extends Application {
    private String content = "<h1>Title</h1> <h2>SubTitle</h2> <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr</p>";

    @Override
    public void start(Stage primaryStage) {
        WebView webView = new WebView();
        webView.getEngine().loadContent(content);

        // Create button.
        Button button = new Button("Select");

        // Button EventHandler.
        button.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                // Work correctly.
                int[] indeices = getSelectedIndices(webView);
                System.out.println(indeices[0] + ", " + indeices[1]);

                // netscape.javascript.JSException: IndexSizeError: The index is not in the allowed range.
                // indeices[0] = 1;
                // indeices[1] = 4;
                setSelectedIndices(webView, indeices);
            }
        });
        // Create VBox and add the button.
        VBox vbox = new VBox(button);

        // Create BorderPane and add VBox.
        BorderPane root = new BorderPane();
        root.setTop(vbox);
        root.setCenter(webView);

        // Create Scene
        Scene scene = new Scene(root, 800, 600);

        // Setting Stage
        primaryStage.setTitle("WebView Példa");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

    public int[] getSelectedIndices(WebView webView) {
        String script = "var selection = window.getSelection();" +
                "if (selection.rangeCount > 0) {" +
                "    var range = selection.getRangeAt(0);" +
                "    var preSelectionRange = range.cloneRange();" +
                "    preSelectionRange.selectNodeContents(document.body);" +
                "    preSelectionRange.setEnd(range.startContainer, range.startOffset);" +
                "    var start = preSelectionRange.toString().length;" +
                "    var end = start + range.toString().length;" +
                "    start + ',' + end;" +
                "} else {" +
                "    '0,0';" +
                "}";

        String result = (String) webView.getEngine().executeScript(script);
        int[] indices = new int[2];
        if (result != null && !result.isEmpty()) {
            String[] splitResult = result.split(",");
            indices[0] = Integer.parseInt(splitResult[0]);
            indices[1] = Integer.parseInt(splitResult[1]);
        } else {
            indices[0] = 0;
            indices[1] = 0;
        }
        return indices;
    }

    public void setSelectedIndices(WebView webView, int[] indices) {
        String script = "function selectText(start, end) {" +
                "    var range = document.createRange();" +
                "    var selection = window.getSelection();" +
                "    range.setStart(document.body.firstChild, start);" +
                "    range.setEnd(document.body.firstChild, end);" +
                "    selection.removeAllRanges();" +
                "    selection.addRange(range);" +
                "}" +
                "selectText(" + indices[0] + ", " + indices[1] + ");";

        webView.getEngine().executeScript(script);
        System.out.println("SELECTED FINSIHED.");
    }
}

I tried to changed the Js script.

String script = "function selectText(start, end) {" +
    "    var node = document.body;" +
    "    var range = document.createRange();" +
    "    var selection = window.getSelection();" +
    "    range.setStart(node, start);" +
    "    range.setEnd(node, end);" +
    "    selection.removeAllRanges();" +
    "    selection.addRange(range);" +
    "}" +
    "selectText(" + indices[0] + ", " + indices[1] + ");";

This way, the method doesn’t always throw errors, but either doesn’t select anything or doesn’t select the specified range