Fixed indexer binding for null target property.

src/Avalonia.Base/Data/IndexerBinding.cs

@@ -1,6 +1,4 @@
-using System;
-
-namespace Avalonia.Data
+namespace Avalonia.Data
 {
     public class IndexerBinding : IBinding
     {
@@ -24,23 +22,7 @@ namespace Avalonia.Data
             object anchor = null,
             bool enableDataValidation = false)
         {
-            var mode = Mode == BindingMode.Default ?
-                targetProperty.GetMetadata(target.GetType()).DefaultBindingMode :
-                Mode;
-
-            switch (mode)
-            {
-                case BindingMode.OneTime:
-                    return InstancedBinding.OneTime(Source.GetObservable(Property));
-                case BindingMode.OneWay:
-                    return InstancedBinding.OneWay(Source.GetObservable(Property));
-                case BindingMode.OneWayToSource:
-                    return InstancedBinding.OneWayToSource(Source.GetSubject(Property));
-                case BindingMode.TwoWay:
-                    return InstancedBinding.TwoWay(Source.GetSubject(Property));
-                default:
-                    throw new NotSupportedException("Unsupported BindingMode.");
-            }
+            return new InstancedBinding(Source.GetSubject(Property), Mode, BindingPriority.LocalValue);
         }
     }
 }