this.Navigation.PushAsync(new Page()); NullReferenceException Error

I am creating a simple app to do some testing before I add the contents that I need and I am getting a NullReferenceException Error. Below is the code that I have so far.

MainPageView
<?xml version="1.0" encoding="utf-8" ?> <ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="MyApp.View.MainPageView"> <StackLayout VerticalOptions="Center"> <Label Text="{Binding Title}" VerticalOptions="Center" HorizontalOptions="Center" /> <Button Text="{Binding ButtonTitle}" Command="{Binding ButtonCommand}" /> </StackLayout> </ContentPage>
`
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;
using MyApp.ViewModel;

namespace MyApp.View
{
public partial class MainPageView
{
public MainPageView()
{
InitializeComponent();
this.BindingContext = new MainPageViewModel();
}
}
}
MainPageViewModel
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
using MyApp.View;
using Xamarin.Forms;

namespace MyApp.ViewModel
{
public class MainPageViewModel : BaseViewModel
{
public string Title { get; set; }
public string ButtonTitle { get; set; }
public ICommand Command { get; set; }

    public MainPageViewModel()
    {
        this.Title = "My Xamarin App";
        this.ButtonTitle = "Click On Me";
        this.ListCommand = new Command(this.ButtonClick);
    }

    public async void ButtonClick()
    {
        await this.Navigation.PushAsync(new ListItemsView());
    }
}

}
ListItemsView and ListItemsViewModel
<?xml version="1.0" encoding="utf-8" ?>

using Xamarin.Forms;

namespace MyApp.View
{
public class ListItemsViewModel
{
public string Title { get; set; }
public ListItemsViewModel()
{
this.Title = "List Items View Page";
}
}
}
`