NulReferenceException on XLabs.Forms.Controls.CheckBoxRenderer.OnElementChanged

NewAlbumPage.xaml

<?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="PixbeeProto.NewAlbumPage"
	Title="Create Album"
	Padding="10, 40, 10, 10"
	xmlns:controls="clr-namespace:XLabs.Forms.Controls;assembly=XLabs.Forms">

	<StackLayout>
		<Label Text="Create a new album!"
	         VerticalOptions="Start"
	         XAlign="Center"
	         IsVisible="true"
	         FontSize="Large"
	         FontAttributes="Bold"
	         TextColor="Gray" />
	    <StackLayout Orientation="Horizontal">
			<Label Text="Title"/>
	    	<Entry WidthRequest = "200"
	    		Text="Great Moment" x:Name="Title" />
	  	</StackLayout>
	    <StackLayout Orientation="Horizontal">
		<ListView x:Name="ListView">
			<ListView.ItemTemplate>
				<DataTemplate>
					<ViewCell>
						<ViewCell.View>
							<controls:CheckBox DefaultText="{Binding}" CheckedText="{Binding}" UncheckedText="{Binding}" HorizontalOptions="FillAndExpand" PropertyChanged="OnCheckedPropertyChanged" />
						</ViewCell.View>
					</ViewCell>
				</DataTemplate>
			</ListView.ItemTemplate>
		</ListView>
		</StackLayout>
		<StackLayout Orientation="Horizontal" HorizontalOptions="Center" VerticalOptions="Center">
			<Button Text="Create!"
				Clicked="OnButtonClicked" />
			<BoxView Color="Transparent" WidthRequest="20" />
			<Button Text="Cancel"
				Clicked="OnButtonClicked" x:Name="cancelButton" />
		</StackLayout>
		<StackLayout>
		<Label Text="Status"
			x:Name="statusLabel"
			Font="Small"
			HorizontalOptions="Center"
			VerticalOptions="CenterAndExpand"
			/>
		</StackLayout>
	</StackLayout>

</ContentPage>

NewAlbumPage.xaml.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

using Xamarin.Forms;
using RestSharp.Portable;
using System.Net.Http;
using System.Linq;

namespace PixbeeProto
{
	public partial class NewAlbumPage : ContentPage
	{
		public List<User> members = new List<User>();

		public NewAlbumPage ()
		{
			InitializeComponent ();

			ListView.ItemsSource = Enum.GetValues(typeof(DayOfWeek)).OfType<DayOfWeek>().Select(c => c.ToString());
		}

		void OnCheckedPropertyChanged(object sender, EventArgs args)
		{
			Console.WriteLine ("checked {0}", args);
		}

		async void OnButtonClicked(object sender, EventArgs args)
		{
			if (sender == cancelButton) {
				await Navigation.PopModalAsync ();
				return;
			}

			statusLabel.Text = "Creating...";

			List<string> memberIds = new List<string>();

			foreach (var member in this.members) {
				memberIds.Add (member.Id);
			}

			Album album = await CreateAlbum (Title.Text, memberIds);

			statusLabel.Text = string.Format ("{0} created.", album.Title);

			await Navigation.PopModalAsync ();

		}

		private static async Task<Album> CreateAlbum(string title, List<string> memberIds) {
			var client = new RestClient(new Uri(String.Format("{0}/api/", UserSession.Instnace.host)));
			var request = new RestRequest ("albums", HttpMethod.Post);
			request.AddHeader ("Authorization", String.Format ("Bearer {0}", UserSession.Instnace.token));
			request.AddQueryParameter ("album[title]", title);
			foreach (var idString in memberIds) {
				request.AddQueryParameter ("album[members]", idString);
			}

			var result = await client.Execute<Album> (request);
			return result.Data;
		}
	}
}