fix failing test

src/Markup/Avalonia.Markup/Markup/Parsers/BindingExpressionGrammar.cs

@@ -183,6 +183,11 @@ namespace Avalonia.Markup.Parsers
 
             var name = r.ParseIdentifier();
 
+            if (name.Length == 0)
+            {
+                throw new ExpressionParseException(r.Position, "Attached Property name expected after '.'.");
+            }
+
             if (r.End || !r.TakeIf(')'))
             {
                 throw new ExpressionParseException(r.Position, "Expected ')'.");

tests/Avalonia.Markup.UnitTests/Parsers/ExpressionObserverBuilderTests_AttachedProperty.cs

@@ -129,7 +129,7 @@ namespace Avalonia.Markup.UnitTests.Parsers
         {
             var data = new Class1();
 
-            Assert.Throws<ExpressionParseException>(() => ExpressionObserverBuilder.Build(data, "(Owner)", typeResolver: _typeResolver));
+            Assert.Throws<ExpressionParseException>(() => ExpressionObserverBuilder.Build(data, "(Owner.)", typeResolver: _typeResolver));
         }
 
         [Fact]