Instantiator.cs

    public static class Instantiator
    {
        static Dictionary<Type, Func<object>> compiledExpressions = new Dictionary<Type, Func<object>>();

        static Dictionary<Tuple<Type, Type>, Func<object>> genericCompiledExpressions = new Dictionary<Tuple<Type, Type>, Func<object>>();

        public static T CreateInstance<T>()
        {
            return (T)CreateInstance(typeof(T));
        }

        public static object CreateInstance(Type typeDefinition)
        {
            Func<object> instance;

            if (!compiledExpressions.TryGetValue(typeDefinition, out instance))
            {
                instance = Expression.Lambda<Func<object>>(Expression.New(typeDefinition)).Compile();

                compiledExpressions.Add(typeDefinition, instance);
            }

            return instance();
        }

        public static object CreateGenericInstance(Type genericTypeDefinition, Type genericParameter)
        {
            Func<object> instance;

            var key = Tuple.Create(genericTypeDefinition, genericParameter);

            if (!genericCompiledExpressions.TryGetValue(key, out instance))
            {
                var genericType = genericTypeDefinition.MakeGenericType(genericParameter);

                instance = instance = Expression.Lambda<Func<object>>(Expression.New(genericType)).Compile();

                genericCompiledExpressions.Add(key, instance);
            }

            return instance();
        }
    }

MenuViewModel.cs

    public class MenuViewModel : ViewModelBase
    {
        Command navigateCommand;
        public Command NavigateCommand => navigateCommand ?? (navigateCommand = new Command(async (navigationKey) => await NavigateAsync((NavigationKeys)navigationKey)));

        public Dictionary<NavigationKeys, INavigationItem<ViewModelBase>> NavigationItems { get; } = new Dictionary<NavigationKeys, INavigationItem<ViewModelBase>>();

        public MenuViewModel()
        {
            NavigationItems.Add(NavigationKeys.Feed, new NavigationItem<FeedViewModel>());
            NavigationItems.Add(NavigationKeys.Friends, new NavigationItem<FriendsViewModel>());
            NavigationItems.Add(NavigationKeys.Groups, new NavigationItem<GroupsViewModel>());
            NavigationItems.Add(NavigationKeys.Settings, new NavigationItem<SettingsViewModel>());
        }

        private async Task NavigateAsync(NavigationKeys navigationKey)
        {
            var navigationItem = NavigationItems[navigationKey];

            if(navigationItem.ViewModel == null)
                navigationItem.ViewModel = Instantiator.CreateInstance(navigationItem.ViewModelType) as ViewModelBase;

            NavigationService.Instance.SwitchDetailPage(navigationItem.ViewModel);
        }
    }

NavigationService.cs

    public class NavigationService : INavigationService
    {
    
        //...
    
        IViewFor<T> InstantiateView<T>(T viewModel) where T : class, IViewModel
        {
            // Figure out what type the view model is
            var viewModelType = viewModel.GetType();

            // look up what type of view it corresponds to
            var viewType = _viewModelViewDictionary[viewModelType];

            // instantiate it
            //var view = (IViewFor<T>)Activator.CreateInstance(viewType);
            var view = (IViewFor<T>)Instantiator.CreateInstance(viewType);

            view.ViewModel = viewModel as T;

            return view;
        }

        IViewFor InstantiateViewForSwitchDetailPage<T>(T viewModel) where T : class, IViewModel
        {
            var viewModelType = viewModel.GetType();

            var viewType = _viewModelViewDictionary[viewModelType];

            var view = (IViewFor)Instantiator.CreateInstance(viewType);

            var viewModelProperty = viewType.GetRuntimeProperty(nameof(IViewFor<T>.ViewModel));

            viewModelProperty?.SetValue(view, viewModel);

            return view;
        }
    }