fix: Fix NRE from x:Bind two-way through converter

Fix NRE when accessing ValueType property in this binding scenario. (Also probably a couple of other would-be NREs as well.)

src/Uno.UI.Tests/Windows_UI_Xaml_Data/xBindTests/Controls/Binding_Converter_TwoWay.xaml

@@ -0,0 +1,48 @@
+<Page
+    x:Class="Uno.UI.Tests.Windows_UI_Xaml_Data.xBindTests.Controls.Binding_Converter_TwoWay"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:Uno.UI.Tests.Windows_UI_Xaml_Data.xBindTests.Controls"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+	  Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+	<Page.Resources>
+		<local:BoolToIntegerConverter x:Key="BoolToIntegerInverseConverter"
+									  IsInverse="True" />
+		<local:InverseBoolConverter x:Key="InverseBoolConverter" />
+	</Page.Resources>
+	<Grid>
+		<Grid.RowDefinitions>
+			<RowDefinition Height="Auto" />
+			<RowDefinition Height="*" />
+		</Grid.RowDefinitions>
+
+		<ListView x:Name="ViewToggleListView"
+				  x:FieldModifier="public"
+				  Margin="0,0,0,8"
+				  SelectedIndex="{x:Bind ViewModel.ShowImages, Mode=TwoWay, Converter={StaticResource BoolToIntegerInverseConverter}, FallbackValue=0}">
+			<ListViewItem Padding="8,1,8,0">
+				<TextBlock Text="Image" />
+			</ListViewItem>
+			<ListViewItem Padding="8,1,8,0">
+				<TextBlock Text="List" />
+			</ListViewItem>
+		</ListView>
+
+		<CheckBox x:Name="BoundCheckBox"
+				  x:FieldModifier="public"
+				  IsChecked="{x:Bind ViewModel.ShowImages, Mode=OneWay}" />
+
+		<Grid Grid.Row="1">
+			<Border x:Name="ImageContent"
+					x:Load="{x:Bind ViewModel.ShowImages}">
+				<TextBlock Text="SHOWING IMAGES" />
+			</Border>
+			<Border x:Name="ListContent"
+					x:Load="{x:Bind ViewModel.ShowImages, Converter={StaticResource InverseBoolConverter}}">
+				<TextBlock Text="SHOWING LIST" />
+			</Border>
+		</Grid>
+	</Grid>
+</Page>

src/Uno.UI.Tests/Windows_UI_Xaml_Data/xBindTests/Controls/Binding_Converter_TwoWay.xaml.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.Tests.Windows_UI_Xaml_Data.xBindTests.Controls
+{
+	/// <summary>
+	/// An empty page that can be used on its own or navigated to within a Frame.
+	/// </summary>
+	public sealed partial class Binding_Converter_TwoWay : Page
+	{
+		public TestViewModel ViewModel { get; }
+
+		public Binding_Converter_TwoWay()
+		{
+			this.InitializeComponent();
+
+			DataContext = ViewModel = new TestViewModel() { ShowImages = true };
+
+			//Loaded += MainPage_Loaded;
+		}
+
+		//private void MainPage_Loaded(object sender, Windows.UI.Xaml.RoutedEventArgs e)
+		//{
+		//}
+
+		public class TestViewModel : INotifyPropertyChanged
+		{
+			public event PropertyChangedEventHandler PropertyChanged;
+
+			public bool ShowImages
+			{
+				get { return _showImages; }
+				set
+				{
+					if (_showImages != value)
+					{
+						_showImages = value;
+						PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(ShowImages)));
+					}
+				}
+			}
+			bool _showImages = false;
+		}
+	}
+}

src/Uno.UI.Tests/Windows_UI_Xaml_Data/xBindTests/Controls/BoolToIntegerConverter.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Windows.UI.Xaml.Data;
+
+namespace Uno.UI.Tests.Windows_UI_Xaml_Data.xBindTests.Controls
+{
+	public class BoolToIntegerConverter : IValueConverter
+	{
+		public bool IsInverse { get; set; }
+
+		public object Convert(object value, Type targetType, object parameter, string language)
+		{
+			var boolValue = value as bool?;
+			if (!boolValue.HasValue)
+			{
+				return null;
+			}
+
+			if (IsInverse)
+			{
+				boolValue = !boolValue.Value;
+			}
+
+			return boolValue.Value ? 1 : 0;
+		}
+
+		public object ConvertBack(object value, Type targetType, object parameter, string language)
+		{
+			var intValue = value as int?;
+			if (!intValue.HasValue)
+			{
+				return null;
+			}
+
+			var result = intValue.Value >= 1;
+
+			if (IsInverse)
+			{
+				result = !result;
+			}
+
+			return result;
+		}
+	}
+}

src/Uno.UI.Tests/Windows_UI_Xaml_Data/xBindTests/Controls/InverseBoolConverter.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Windows.UI.Xaml.Data;
+
+namespace Uno.UI.Tests.Windows_UI_Xaml_Data.xBindTests.Controls
+{
+	public class InverseBoolConverter : IValueConverter
+	{
+		public object Convert(object value, System.Type targetType, object parameter, string language)
+		{
+			if (value is bool)
+			{
+				return !(bool)value;
+			}
+
+			return null;
+		}
+
+		public object ConvertBack(object value, System.Type targetType, object parameter, string language)
+		{
+			if (value is bool)
+			{
+				return !(bool)value;
+			}
+
+			return null;
+		}
+	}
+}

src/Uno.UI.Tests/Windows_UI_Xaml_Data/xBindTests/Given_xBind_Binding.cs

@@ -142,7 +142,7 @@ public void When_DataTemplate_TwoWay_Nested()
 
 			Assert.AreEqual(2, rootData.Model.MyIntProperty);
 		}
-		
+
 		[TestMethod]
 		public void When_Object_TwoWay()
 		{
@@ -344,6 +344,31 @@ public void When_Converter()
 			Assert.AreEqual("v:Uno.UI.Tests.Windows_UI_Xaml_Data.xBindTests.Controls.Binding_Converter p:test", SUT.myTextBlock2.Text);
 		}
 
+		[TestMethod]
+		public void When_Converter_TwoWay()
+		{
+			var SUT = new Binding_Converter_TwoWay();
+
+			SUT.ForceLoaded();
+
+			ListView list = SUT.ViewToggleListView;
+
+			CheckBox cb = SUT.BoundCheckBox;
+
+			Assert.AreEqual(0, list.SelectedIndex);
+			Assert.IsTrue(cb.IsChecked.Value);
+
+			list.SelectedItem = list.Items[1];
+
+			Assert.AreEqual(1, list.SelectedIndex);
+			 Assert.IsFalse(cb.IsChecked.Value);
+
+			list.SelectedItem = list.Items[0];
+
+			Assert.AreEqual(0, list.SelectedIndex);
+			Assert.IsTrue(cb.IsChecked.Value);
+		}
+
 		[TestMethod]
 		public void When_ConverterParameter()
 		{
@@ -479,7 +504,7 @@ public void When_DefaultBindingMode_TwoWay()
 			Assert.AreEqual("TwoWay updated 5", SUT.Default_TwoWay_OneWay_Property);
 			Assert.AreEqual("TwoWay updated 9", SUT.Default_TwoWay_TwoWay_Property);
 		}
-		
+
 		[TestMethod]
 		public void When_DefaultBindingMode_Nested()
 		{