romrepomgr/RomRepoMgr/App.xaml.cs

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using RomRepoMgr.ViewModels;
using RomRepoMgr.Views;

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

        public override void OnFrameworkInitializationCompleted()
        {
            if(ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
                desktop.MainWindow = new MainWindow
                {
                    DataContext = new MainWindowViewModel()
                };

            base.OnFrameworkInitializationCompleted();
        }
    }
}