Fixed BindingExtension.TargetNullValue default value.

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/BindingExtension.cs

@@ -87,7 +87,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
         public string StringFormat { get; set; }
 
         public RelativeSource RelativeSource { get; set; }
-        
-        public object TargetNullValue { get; set; }
+
+        public object TargetNullValue { get; set; } = AvaloniaProperty.UnsetValue;
     }
 }

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/BindingExtensionTests.cs

@@ -38,7 +38,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
                 Assert.Equal("foobar", textBlock.Text);
             }
         }
-        
+
         [Fact]
         public void BindingExtension_Binds_To_TargetNullValue()
         {
@@ -65,6 +65,28 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
             }
         }
 
+        [Fact]
+        public void BindingExtension_TargetNullValue_UnsetByDefault()
+        {
+            using (StyledWindow())
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <TextBlock Name='textBlock' IsVisible='{Binding Foo, Converter={x:Static ObjectConverters.IsNotNull}}'/>
+</Window>";
+
+                var loader = new AvaloniaXamlLoader();
+                var window = (Window)loader.Load(xaml);
+                var textBlock = window.FindControl<TextBlock>("textBlock");
+
+                window.DataContext = new FooBar();
+                window.Show();
+
+                Assert.Equal(false, textBlock.IsVisible);
+            }
+        }
+
         private class FooBar
         {
             public object Foo { get; } = null;