Merge pull request #5198 from AvaloniaUI/fixes/nre-with-indexer

Fix: NullReferenceException when using TwoWay binding with indexer to null DataContext

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

@@ -15,7 +15,8 @@ namespace Avalonia.Data.Core
 
         private bool ShouldNotSet(object value)
         {
-            if (PropertyType == null)
+            var propertyType = PropertyType;
+            if (propertyType == null)
             {
                 return false;
             }
@@ -37,7 +38,7 @@ namespace Avalonia.Data.Core
                 return false;
             }
 
-            if (PropertyType.IsValueType)
+            if (propertyType.IsValueType)
             {
                 return lastValue.Equals(value);
             }

src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs

@@ -129,7 +129,10 @@ namespace Avalonia.Markup.Parsers.Nodes
         {
             get
             {
-                Target.TryGetTarget(out object target);
+                if (!Target.TryGetTarget(out object target))
+                {
+                    return null;
+                }
 
                 return GetIndexer(target.GetType().GetTypeInfo())?.PropertyType;
             }

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Binding.cs

@@ -4,6 +4,8 @@ using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using System.Threading;
 using System.Threading.Tasks;
+
+using Avalonia.Controls;
 using Avalonia.Data;
 using Avalonia.Logging;
 using Avalonia.Platform;
@@ -797,6 +799,24 @@ namespace Avalonia.Base.UnitTests
             Assert.False(source.SetterCalled);
         }
 
+        [Fact]
+        public void TwoWay_Binding_Should_Not_Fail_With_Null_DataContext()
+        {
+            var target = new TextBlock();
+            target.DataContext = null;
+
+            target.Bind(TextBlock.TextProperty, new Binding("Missing", BindingMode.TwoWay));
+        }
+
+        [Fact]
+        public void TwoWay_Binding_Should_Not_Fail_With_Null_DataContext_Indexer()
+        {
+            var target = new TextBlock();
+            target.DataContext = null;
+
+            target.Bind(TextBlock.TextProperty, new Binding("[0]", BindingMode.TwoWay));
+        }
+
         [Fact]
         public void Disposing_Completed_Binding_Does_Not_Throw()
         {