Convert HighCharts .Net Core 2.1 Application to .Net 5

Question

I am trying to use HighCharts in a .Net5 Application. All of the examples that they provide appear to be written for .Net Core 2.1. As an example, here is the code that HighSoft provides for creating a bubble chart. I pasted this into my view. I added the appropriate code in my controller. I added the HighSoft.HighCharts and Newtonsoft.Json packages thru Nuget.

@using Highsoft.Web.Mvc.Charts;
@using Highsoft.Web.Mvc.Charts.Rendering;

<h1>@ViewData["Title"]</h1>
<div>
    @{ var chartOptions = new Highcharts
        {
            Chart = new Highsoft.Web.Mvc.Charts.Chart
            {
                ZoomType = ChartZoomType.Xy
            },
            Title = new Title
            {
                Text = "Highcharts Bubbles"
            },
            Series = new List<Series>
            {
                new BubbleSeries
                {
                    Data = @ViewData["bubble1"] as List<BubbleSeriesData>
                },
                new BubbleSeries
                {
                    Data = @ViewData["bubble2"] as List<BubbleSeriesData>
                },
                new BubbleSeries
                {
                    Data = @ViewData["bubble3"] as List<BubbleSeriesData>
                }
            }
        };
     chartOptions.ID = "chart";
     var renderer = new HighchartsRenderer(chartOptions);
     }

    @Html.Raw(renderer.RenderHtml())
</div>

Under .Net Core 2.1, there are no errors. When I switch to .Net5 (or .Net Core 3.1 for that matter). the following lines throw errors:

Data = @ViewData["bubble1"] as List<BubbleSeriesData>
Data = @ViewData["bubble2"] as List<BubbleSeriesData>
Data = @ViewData["bubble3"] as List<BubbleSeriesData>

Intellisense is telling me "The BubbleSeriesData element was not closed. All elements must be self closing or having a matching end tag.

How do I change those lines so they work in .Net5?