[C] clear _targetObject (#18398)

Avoid NRE on reapplying.

- fixes #18123

src/Controls/src/Core/MultiBinding.cs

@@ -238,6 +238,7 @@ internal override void Unapply(bool fromBindingContextChanged = false)
 
 				_bpProxies = null;
 				_proxyObject = null;
+				_targetObject = null;
 			}
 
 			base.Unapply(fromBindingContextChanged: fromBindingContextChanged);

src/Controls/tests/Xaml.UnitTests/Issues/Maui18123.xaml

@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             xmlns:local="clr-namespace:Microsoft.Maui.Controls.Xaml.UnitTests"
+             x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Maui18123">
+    <ContentPage.Resources>
+        <local:Maui18123MultiValueConverter x:Key="MultiValueConverter"/>
+    </ContentPage.Resources>
+    <StackLayout>
+        <Button
+            Text="Delete" WidthRequest="100"
+            HorizontalOptions="Center" 
+            Command="{Binding TestCommand}"
+            x:Name="deleteBtn">
+            <Button.CommandParameter>
+                <MultiBinding Converter="{StaticResource MultiValueConverter}">
+                    <Binding Source="Dummy"/>
+                    <Binding Source="Edit"/>
+                </MultiBinding>
+            </Button.CommandParameter>
+            <Button.Triggers>
+                <DataTrigger TargetType="Button" Binding="{Binding Mode}" Value="Edit">
+                    <Setter Property="Text" Value="CANCEL" />
+                    <Setter Property="CommandParameter">
+                        <Setter.Value>
+                            <MultiBinding Converter="{StaticResource MultiValueConverter}">
+                                <Binding Source="Cancel"/>
+                            </MultiBinding>
+                        </Setter.Value>
+                    </Setter>
+                </DataTrigger>
+            </Button.Triggers>
+        </Button>
+
+        <Button
+            Text="Edit" WidthRequest="100" 
+            HorizontalOptions="Center" 
+            Command="{Binding TestCommand}"
+            x:Name="editBtn">
+            <Button.CommandParameter>
+                <MultiBinding Converter="{StaticResource MultiValueConverter}">
+                    <Binding Source="Dummy"/>
+                    <Binding Source="Edit"/>
+                </MultiBinding>
+            </Button.CommandParameter>
+            <Button.Triggers>
+                <DataTrigger TargetType="Button" Binding="{Binding Mode}" Value="Edit">
+                    <Setter Property="Text" Value="SUBMIT" />
+                </DataTrigger>
+                <DataTrigger TargetType="Button" Binding="{Binding Mode}" Value="Create">
+                    <Setter Property="Text" Value="SUBMIT" />
+                    <Setter Property="CommandParameter" Value="Edit"/>
+                </DataTrigger>
+            </Button.Triggers>
+        </Button>
+    </StackLayout>
+</ContentPage>

src/Controls/tests/Xaml.UnitTests/Issues/Maui18123.xaml.cs

@@ -0,0 +1,96 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Globalization;
+using System.Linq;
+using Microsoft.Maui.ApplicationModel;
+using Microsoft.Maui.Controls.Core.UnitTests;
+using Microsoft.Maui.Controls.Shapes;
+using Microsoft.Maui.Devices;
+using Microsoft.Maui.Dispatching;
+
+using Microsoft.Maui.Graphics;
+using Microsoft.Maui.UnitTests;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public class Maui18123MultiValueConverter : IMultiValueConverter
+{
+	public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
+		=> values?.ToArray();
+
+	public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
+		=> throw new NotImplementedException();
+}
+
+public partial class Maui18123 : ContentPage
+{
+
+	public Maui18123() => InitializeComponent();
+
+	public Maui18123(bool useCompiledXaml)
+	{
+		//this stub will be replaced at compile time
+	}
+
+	[TestFixture]
+	class Test
+	{
+		[SetUp]
+		public void Setup()
+		{
+			Application.SetCurrentApplication(new MockApplication());
+			DispatcherProvider.SetCurrent(new DispatcherProviderStub());
+		}
+
+
+		[TearDown] public void TearDown() => AppInfo.SetCurrent(null);
+
+		[Test]
+		public void MultiBindingShouldNotThrow([Values(false, true)] bool useCompiledXaml)
+		{
+			if (useCompiledXaml)
+				Assert.DoesNotThrow(() => MockCompiler.Compile(typeof(Maui18123)));
+
+			var page = new Maui18123(useCompiledXaml);
+			page.BindingContext = new Maui18123VM();
+			page.editBtn.SendClicked();
+			Assert.That(page.editBtn.Text, Is.EqualTo("SUBMIT"));
+			Assert.That(page.deleteBtn.Text, Is.EqualTo("CANCEL"));
+			page.deleteBtn.SendClicked();
+			Assert.That(page.editBtn.Text, Is.EqualTo("Edit"));
+			Assert.That(page.deleteBtn.Text, Is.EqualTo("Delete"));
+		}
+	}
+
+	public partial class Maui18123VM : BindableObject
+	{
+		string _mode = "";
+		public string Mode
+		{
+			get { return _mode; }
+			set
+			{
+				_mode = value;
+				OnPropertyChanged();
+			}
+		}
+
+		Command testCommand;
+		public Command TestCommand => testCommand ??= new Command(Test);
+
+		void Test(object parameter)
+		{
+			var value = string.Empty;
+
+			if (parameter is object[] parameters)
+				value = parameters.LastOrDefault()?.ToString();
+			else
+				value = parameter.ToString();
+
+			Mode = value;
+		}
+
+	}
+}