UWP - CheckBox or Button is not visible

Using

Jetbrains Rider

Universal Windows Platform

Problem

The window is visible, but the buttons are not visible.

Question

How can I fix the problem?

Source code

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

namespace UWP_TEST1
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>

    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
            
            StackPanel stackPanel1 = new StackPanel();
            CheckBox checkBox1 = new CheckBox();
            checkBox1.Content = "I agree to the terms of service.";
            
            stackPanel1.Children.Add(checkBox1);
            
        }
    }
}