Avalonia/AvaloniaApplication3/ViewModels/ViewModelBase.cs

using Avalonia.Controls;
using AvaloniaApplication3.Views;
using ReactiveUI;

namespace AvaloniaApplication3.ViewModels
{
    public class ViewModelBase : ReactiveObject
    {
        UserControl? mainAcc;
        public UserControl? MainAcc { get => mainAcc; set => this.RaiseAndSetIfChanged(ref mainAcc, value); }
    }
}