Having issue in Volley [Android]

Question

I'm trying to retrieve data from a third-party API but I'm having this issue down below:

 Process: com.main.instareeldownloader, PID: 8483
com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1 path $
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:225)

This is where I'm trying to retrieve data.

StringRequest request = new StringRequest(URL, response -> {
        GsonBuilder gsonBuilder = new GsonBuilder();
        Gson gson = gsonBuilder.create();

        MainUrl mainUrl = gson.fromJson(response, MainUrl.class);
        reeelUrl = mainUrl.getItems();

        Log.e("Data",reeelUrl);
        uri2 = Uri.parse(reeelUrl);
        videoView.setMediaController(mediaController);
        videoView.setVideoURI(uri2);
        videoView.requestFocus();
        videoView.start();
    }, error -> Toast.makeText(getContext(), "Not able to Fetch.", Toast.LENGTH_SHORT).show());

    RequestQueue queue = Volley.newRequestQueue(getContext());
    queue.add(request);

This is my MainUrl Model

String items;
//private MainReel items;

public MainUrl(String items) {
    this.items = items;
}

public String getItems() {
    return items;
}

public void setItems(String items) {
    this.items = items;
}

@Override
public String toString() {
    return "MainUrl{" +
            "items='" + items + '\'' +
            '}';
}

JSON format

From the JSON try to retrieve this:

items -> video_versions -> url

I'm confused why an error is occurring for.