asp.net core razor pages - cascade dropdownlists

Question

I tried to implement the example here 

 

https://www.c-sharpcorner.com/article/creating-simple-cascading-dropdownlist-in-asp-net-core-mvc-with-new-tag-helpers/

 

into my Razor page project but the second dropdown doesn't get any data. What I did is

 

View Page including script

 

<div class="col-2">
    <label for="PTWReviewD.Category" class="form-control badge-primary">PTW Review Category</label>
    <select asp-for="PTWReviewD.Category" class="form-control border-warning" asp-items="Model.PTWRCList">
        <option value="">--Select PTW Review Category--</option>
    </select>
    <span asp-validation-for="PTWReviewD.Category" class="text-danger"></span>
</div>
<div class="col-2">
    <label for="PTWReviewD.SubCategory" class="form-control badge-primary">PTW Review Subcategory</label>
    <select asp-for="PTWReviewD.SubCategory" class="form-control border-warning" asp-items="@(new SelectList(string.Empty, "RSubCategoryID", "RSubCategoryS"))">
        <option value="">--Select PTW Review Subcategory--</option>
    </select>
    <span asp-validation-for="PTWReviewD.SubCategory" class="text-danger"></span>
</div>

@Html.AntiForgeryToken()

@section scripts{

<script>
    $(function () {
        $("#Category").on("change", function func() {
            var url = '@Url.Content("~/")' + "PTWReview/OnGetSubCategoriesAsync";
            var idSource = "#RCategoryID";

            $.getJSON(url, { RCategoryID: $(idSource).val() }, function (data) {
                var items = '';
                $("#SubCategory").empty();
                $.each(data, function (i, ptwrsclist) {
                    items += "<option value='" + ptwrsclist.RSubCategoryID + "'>" + ptwrsclist.RSubCategoryS + "</option>";
                });
                $("#SubCategory").html(items);
            });
        });
    });
</script>

}

Getting the data with

 

public async Task<IActionResult> OnGetAsync(int ptwNoId)
        {

            //PTWRCategory
            PTWRCList = await _context.PTWRCategories.Select(a =>
                                       new SelectListItem
                                       {
                                           Value = a.RCategoryID.ToString(),
                                           Text = a.RCategoryS
                                       }).ToListAsync();


            return Page();
        }

        public async Task<JsonResult> OnGetSubCategoriesAsync(int RCategoryID)
        {
            PTWRSCList = await _context.PTWRSubCategories.Where(a => a.RCategoryID == RCategoryID).Select(a =>
                                       new SelectListItem
                                       {
                                           Value = a.RSubCategoryID.ToString(),
                                           Text = a.RSubCategoryS
                                       }).ToListAsync();

            return new JsonResult(new SelectList(PTWRSCList, "RSubCategoryID", "RSubCategoryS"));

        }

The first ddl is filled with data but the second is not and I have no errors. What am I doing wrong?