Fix stream bindings.

src/Markup/Avalonia.Markup.Xaml/XamlIl/CompilerExtensions/XamlIlBindingPathHelper.cs

@@ -80,7 +80,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                         appendNotNode = !appendNotNode;
                         break;
                     case BindingExpressionGrammar.StreamNode _:
-                        var observableType = targetType.GetAllInterfaces().FirstOrDefault(i => i.GenericTypeDefinition.Equals(context.Configuration.TypeSystem.FindType("System.IObservable`1")));
+                        var observableType = targetType.GetAllInterfaces().FirstOrDefault(i => i.GenericTypeDefinition?.Equals(context.Configuration.TypeSystem.FindType("System.IObservable`1")) ?? false);
                         if (observableType != null)
                         {
                             nodes.Add(new XamlIlStreamObservablePathElementNode(observableType.GenericArguments[0]));

src/Markup/Avalonia.Markup/Data/BindingBase.cs

@@ -13,7 +13,7 @@ using Avalonia.VisualTree;
 
 namespace Avalonia.Data
 {
-    public abstract class BindingBase
+    public abstract class BindingBase : IBinding
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="Binding"/> class.

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

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Text;
+using System.Threading.Tasks;
 using Avalonia.Controls;
 using Avalonia.Markup.Data;
 using Avalonia.UnitTests;
@@ -38,10 +39,41 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
                 Assert.Equal(dataContext.StringProperty, textBlock.Text);
             }
         }
+
+        [Fact]
+        public void ResolvesStreamBindingCorrectly()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'
+        x:DataContextType='{x:Type local:TestDataContext}'>
+    <TextBlock Text='{CompiledBinding TaskProperty^}' Name='textBlock' />
+</Window>";
+                var loader = new AvaloniaXamlLoader();
+                var window = (Window)loader.Load(xaml);
+                var textBlock = window.FindControl<TextBlock>("textBlock");
+
+                DelayedBinding.ApplyBindings(textBlock);
+
+                var dataContext = new TestDataContext
+                {
+                    TaskProperty = Task.FromResult("foobar")
+                };
+
+                window.DataContext = dataContext;
+
+                Assert.Equal(dataContext.TaskProperty.Result, textBlock.Text);
+            }
+        }
     }
 
     public class TestDataContext
     {
         public string StringProperty { get; set; }
+
+        public Task<string> TaskProperty { get; set; }
     }
 }