bfr/BFR/App.xaml.cs

using Avalonia;
using Avalonia.Markup.Xaml;
using Avalonia.Controls.ApplicationLifetimes;

namespace BFR
{
    public class App : Application
    {
        public override void Initialize() =>
            AvaloniaXamlLoader.Load(this);

        public override void OnFrameworkInitializationCompleted()
        {
            switch (ApplicationLifetime)
            {
                case IClassicDesktopStyleApplicationLifetime desktop:
                    desktop.MainWindow = new MainWindow();
                    break;
            }
            base.OnFrameworkInitializationCompleted();
        }
    }
}