[X] resilient to unyped x:Name (#5552)

Allow the previewer to recover from an x:Name attribute on a element
with unresolved type.

- fixes #5546
- closes #5547

Xamarin.Forms.Xaml.UnitTests/DesignTimeLoaderTests.cs

@@ -627,6 +627,25 @@ namespace Xamarin.Forms.Xaml.UnitTests
 			Assert.That(clrNamespace, Is.EqualTo("my.namespace"));
 			Assert.That(typeName, Is.EqualTo("MissingType"));
 		}
+
+		[Test]
+		public void IgnoreNamedMissingTypeException()
+		{
+			var xaml = @"
+					<ContentPage xmlns=""http://xamarin.com/schemas/2014/forms""
+						xmlns:x=""http://schemas.microsoft.com/winfx/2009/xaml""
+						xmlns:local=""clr-namespace:Xamarin.Forms.Xaml.UnitTests;assembly=Xamarin.Forms.Xaml.UnitTests"">
+						<StackLayout>
+							<local:Missing x:Name=""MyName"" />
+							<Button x:Name=""button"" />
+							<Button x:Name=""button"" />
+						</StackLayout>
+					</ContentPage>";
+			var exceptions = new List<Exception>();
+			Xamarin.Forms.Internals.ResourceLoader.ExceptionHandler = exceptions.Add;
+			Assert.DoesNotThrow(() => XamlLoader.Create(xaml, true));
+			Assert.That(exceptions.Count, Is.GreaterThan(1));
+		}
 	}
 
 	public class InstantiateThrows

Xamarin.Forms.Xaml/RegisterXNamesVisitor.cs

@@ -30,18 +30,29 @@ namespace Xamarin.Forms.Xaml
 		{
 			if (!IsXNameProperty(node, parentNode))
 				return;
-			try
-			{
+
+			try {
 				((IElementNode)parentNode).Namescope.RegisterName((string)node.Value, Values[parentNode]);
 			}
-			catch (ArgumentException ae)
-			{
+			catch (ArgumentException ae) {
 				if (ae.ParamName != "name")
 					throw ae;
-				throw new XamlParseException($"An element with the name \"{(string)node.Value}\" already exists in this NameScope", node);
+				var xpe = new XamlParseException($"An element with the name \"{(string)node.Value}\" already exists in this NameScope", node);
+				if (Context.ExceptionHandler != null) {
+					Context.ExceptionHandler(xpe);
+					return;
+				}
+				throw xpe;
 			}
-			var element = Values[parentNode] as Element;
-			if (element != null)
+			catch (KeyNotFoundException knfe) {
+				if (Context.ExceptionHandler != null) {
+					Context.ExceptionHandler(knfe);
+					return;
+				}
+				throw knfe;
+			}
+
+			if (Values[parentNode] is Element element)
 				element.StyleId = element.StyleId ?? (string)node.Value;
 		}