!B Fix expression nodes not knowing if last value is actually null or dead.

src/Avalonia.Base/Data/Core/ExpressionNode.cs

@@ -8,9 +8,13 @@ namespace Avalonia.Data.Core
     public abstract class ExpressionNode
     {
         private static readonly object CacheInvalid = new object();
+
         protected static readonly WeakReference<object> UnsetReference = 
             new WeakReference<object>(AvaloniaProperty.UnsetValue);
 
+        protected static readonly WeakReference<object> NullReference =
+            new WeakReference<object>(null);
+
         private WeakReference<object> _target = UnsetReference;
         private Action<object> _subscriber;
         private bool _listening;
@@ -98,7 +102,7 @@ namespace Avalonia.Data.Core
 
             if (notification == null)
             {
-                LastValue = new WeakReference<object>(value);
+                LastValue = value != null ? new WeakReference<object>(value) : NullReference;
 
                 if (Next != null)
                 {
@@ -111,7 +115,7 @@ namespace Avalonia.Data.Core
             }
             else
             {
-                LastValue = new WeakReference<object>(notification.Value);
+                LastValue = notification.Value != null ? new WeakReference<object>(notification.Value) : NullReference;
 
                 if (Next != null)
                 {

src/Avalonia.Base/Data/Core/SettableNode.cs

@@ -29,6 +29,11 @@ namespace Avalonia.Data.Core
 
             if (!isLastValueAlive)
             {
+                if (value == null && LastValue == NullReference)
+                {
+                    return true;
+                }
+
                 return false;
             }
 

tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs

@@ -216,7 +216,7 @@ namespace Avalonia.Markup.UnitTests.Data
             // When running tests under NCrunch, NCrunch replaces the standard StackOverflowException
             // with its own, which will be caught by our code. Detect the stackoverflow anyway, by
             // making sure the target property was only set once.
-            Assert.Equal(1, source.FooSetCount);
+            Assert.Equal(2, source.FooSetCount);
         }
 
         [Fact]