Execute JavaScript in Android Kotlin WebView to find an element's coordinates on the screen

I have an Android app which I am creating with the Kotlin language. I am trying to get a specific <div> element within the webpage I am displaying and then put the coordinates of the element into Kotlin variables. In my case, I'm displaying a YouTube page and trying to get the coordinates of the video player (I will be later displaying a button over it). I'm using the following code to initialize my WebView:

mainWebView.webViewClient = CustomWebViewClient(...) // This custom class doesn't do anything other than check if the user leaves the YT website.
mainWebView.webChromeClient = WebChromeClient()
mainWebView.settings.javaScriptEnabled = true
mainWebView.settings.domStorageEnabled = true

Then, I load the page:

mainWebView.loadUrl("https://www.youtube.com/watch?app=desktop&v=lh7WIW3pcso")

Next, I execute some JavaScript on the click of a specific button and display the result. I tested the JavaScript code in my browser's console and it worked fine. (It gave the result of {"x":0,"y":56}.)

mainWebView.evaluateJavascript(
    "(function() { var element = document.getElementById('full-bleed-container'); var rect = element.getBoundingClientRect(); return JSON.stringify({ x: rect.left, y: rect.top }); })();"
) { result ->

    if (result == "null") {
        Toast.makeText(this, "NULL!", Toast.LENGTH_SHORT).show()
    } else {
        val coordinates = JSONObject(result)
        val x = coordinates.getDouble("x")
        val y = coordinates.getDouble("y")
        Toast.makeText(this, "Coordinates: X: $x, Y: $y", Toast.LENGTH_SHORT).show()
    }
}

However, my app crashes with the following exception: org.json.JSONException: Value {"x":0,"y":56} of type java.lang.String cannot be converted to JSONObject.

I also checked to see if the page wasn't done loading with the following code (and I received the Toast saying it was done):

    override fun onPageFinished(view: WebView?, url: String?) {
        super.onPageFinished(view, url)

        Toast.makeText(context, "Loaded '$url'!", Toast.LENGTH_SHORT).show()
    }

Why isn't the JSON response string being converted into the JSONObject successfully? Not sure what's going on here, but I need to get the coordinates in such a format that I can display a button over that <div> element.

EDIT----

Ok, so someone responded and linked me to this post which was kinda helpful (the only helpful post I found was this answer as all the rest showed things I'd tried already). So I tried Klaxon, and here's my code:

// For parsing
val coordinates = Klaxon().parse<JSONCoordinatesResponse>(result)

// Class that it puts the output into
class JSONCoordinatesResponse (val x: Int, val y: Int)

But it still throws an error when parsing the string:

java.lang.ClassCastException: java.lang.String cannot be cast to com.beust.klaxon.JsonObject

How can I parse this string into a Kotlin variables?