How can I use a Checkbox as column editor in a GridPanel?

Other components seem to work fine, but the Checkbox appears disabled.

I started from your GridPanel sample and here it is:

GridPanel.cshtml

@page  "{handler?}"
@model BBros.Samples.Pages.GridPanelModel
@{
    ViewData["Title"] = "GridPanel";
}
<ext-section target="Main">
    <ext-container region="Center" scrollable="true" paddingAsString="30 20 30 50">
        <items>
            <ext-gridPanel title="GridPanel"
                           width="960"
                           height="640"
                           frame="true">
                <store>
                    <ext-store data="Model.GridData" pageSize="10">
                        <fields>
                            <ext-dataField name="company" />
                            <ext-numberDataField name="price" />
                            <ext-numberDataField name="change" />
                            <ext-numberDataField name="pctChange" />
                            <ext-dateDataField name="lastChange" />
                            <ext-booleanDataField name="really" />
                        </fields>
                        <proxy>
                            <ext-memoryProxy enablePaging="true" />
                        </proxy>
                    </ext-store>
                </store>
                <columns>
                    <ext-column text="Company" dataIndex="company" flex="1" editorModel="new TextField()" />
                    <ext-column text="Price" dataIndex="price" renderer="Ext.util.Format.euroMoney" />
                    <ext-column text="Change" dataIndex="change" renderer="change" />
                    <ext-column text="Change %" dataIndex="pctChange" renderer="pctChange" />
                    <ext-dateColumn text="Last Updated" dataIndex="lastChange" width="150" editorModel="new DateField()" />
                    <ext-checkColumn text="Really?" dataIndex="really" width="50" editorModel="new Checkbox()" />
                </columns>
                <plugins>
                    <ext-cellEditingPlugin />
                </plugins>
                <bbar>
                    <ext-pagingToolbar />
                </bbar>
            </ext-gridPanel>
        </items>
    </ext-container>
</ext-section>
GridPanel.cshtml.cs

using Microsoft.AspNetCore.Mvc.RazorPages;
using System;
using System.Collections.Generic;

namespace BBros.Samples.Pages
{
    public class GridPanelModel : PageModel
    {
        public List<object> GridData { get; set; }

        public void OnGet()
        {
            var now = DateTime.Now.ToString("yyyy-MM-dd hh:mm:tt");

            this.GridData ??= new List<object>
            {
                new object[] { "3m Co", 71.72, 0.02, 0.03, now, true },
                new object[] { "Alcoa Inc", 29.01, 0.42, 1.47, now, true },
                new object[] { "Altria Group Inc", 83.81, 0.28, 0.34, now, true },
                new object[] { "American Express Company", 52.55, 0.01, 0.02, now, true },
                new object[] { "American International Group, Inc.", 64.13, 0.31, 0.49, now, false },
                new object[] { "AT&T Inc.", 31.61, -0.48, -1.54, now, true },
                new object[] { "Boeing Co.", 75.43, 0.53, 0.71, now, true },
                new object[] { "Caterpillar Inc.", 67.27, 0.92, 1.39, now, true },
                new object[] { "Citigroup, Inc.", 49.37, 0.02, 0.04, now, true },
                new object[] { "E.I. du Pont de Nemours and Company", 40.48, 0.51, 1.28, now, true },
                new object[] { "Exxon Mobil Corp", 68.1, -0.43, -0.64, now, true },
                new object[] { "General Electric Company", 34.14, -0.08, -0.23, now, true },
                new object[] { "General Motors Corporation", 30.27, 1.09, 3.74, now, true },
                new object[] { "Hewlett-Packard Co.", 36.53, -0.03, -0.08, now, true },
                new object[] { "Honeywell Intl Inc", 38.77, 0.05, 0.13, now, true },
                new object[] { "Intel Corporation", 19.88, 0.31, 1.58, now, true },
                new object[] { "International Business Machines", 81.41, 0.44, 0.54, now, true },
                new object[] { "Johnson & Johnson", 64.72, 0.06, 0.09, now, true },
                new object[] { "JP Morgan & Chase & Co", 45.73, 0.07, 0.15, now, false },
                new object[] { "McDonald's Corporation", 36.76, 0.86, 2.40, now, true },
                new object[] { "Merck & Co., Inc.", 40.96, 0.41, 1.01, now, true },
                new object[] { "Microsoft Corporation", 25.84, 0.14, 0.54, now, true },
                new object[] { "Pfizer Inc", 27.96, 0.4, 1.45, now, true },
                new object[] { "The Coca-Cola Company", 45.07, 0.26, 0.58, now, true },
                new object[] { "The Home Depot, Inc.", 34.64, 0.35, 1.02, now, true },
                new object[] { "The Procter & Gamble Company", 61.91, 0.01, 0.02, now, true },
                new object[] { "United Technologies Corporation", 63.26, 0.55, 0.88, now, true },
                new object[] { "Verizon Communications", 35.57, 0.39, 1.11, now, true },
                new object[] { "Wal-Mart Stores, Inc.", 45.45, 0.73, 1.63, now, true }
            };
        }
    }
}
Thanks!