presence_new/Presence.Desktop/ViewModels/MainWindowViewModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;
using ReactiveUI;

namespace Presence.Desktop.ViewModels
{
    public class MainWindowViewModel: ViewModelBase, IScreen
    {
        public RoutingState Router { get; } = new RoutingState();

    public MainWindowViewModel(IServiceProvider serviceProvider)
    {
        var groupViewModel = serviceProvider.GetRequiredService<GroupViewModel>();
        Router.Navigate.Execute(groupViewModel);
    }
    }
}