[previewer] resilience on createvalue (#5369)

- fixes #5171

Xamarin.Forms.Xaml.UnitTests/DesignTimeLoaderTests.cs

@@ -51,7 +51,7 @@ namespace Xamarin.Forms.Xaml.UnitTests
 		}
 
 		[Test]
-		public void ContentPageWithMissingType()
+		public void ContentPageWithMissingTypeMockviewReplacement()
 		{
 			XamlLoader.FallbackTypeResolver = (p, type) => type ?? typeof(MockView);
 
@@ -68,6 +68,28 @@ namespace Xamarin.Forms.Xaml.UnitTests
 			Assert.That(page.Content, Is.TypeOf<MockView>());
 		}
 
+		[Test]
+		public void ContentPageWithMissingTypeNoReplacement()
+		{
+			XamlLoader.FallbackTypeResolver = (p, type) => type;
+
+			var xaml = @"
+					<ContentPage xmlns=""http://xamarin.com/schemas/2014/forms""
+						xmlns:local=""clr-namespace:MissingNamespace;assembly=MissingAssembly""
+						xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"">
+						<ContentPage.Content>
+							<local:MyCustomButton Foo=""Bar"">
+								<local:MyCustomButton.Qux>
+									<Label />
+								</local:MyCustomButton.Qux>
+							</local:MyCustomButton>
+						</ContentPage.Content>
+					</ContentPage>";
+
+			var page = (ContentPage)XamlLoader.Create(xaml, true);
+			Assert.That(page.Content, Is.Null);
+		}
+
 		[Test]
 		public void MissingTypeWithKnownProperty()
 		{

Xamarin.Forms.Xaml/ApplyPropertiesVisitor.cs

@@ -36,13 +36,15 @@ namespace Xamarin.Forms.Xaml
 		public bool StopOnResourceDictionary { get; }
 		public bool VisitNodeOnDataTemplate => true;
 		public bool SkipChildren(INode node, INode parentNode) => false;
-		public bool IsResourceDictionary(ElementNode node) => typeof(ResourceDictionary).IsAssignableFrom(Context.Types[node]);
+		public bool IsResourceDictionary(ElementNode node) => Context.Types.TryGetValue(node, out var type) && typeof(ResourceDictionary).IsAssignableFrom(type);
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
 			var parentElement = parentNode as IElementNode;
 			var value = Values [node];
-			var source = Values [parentNode];
+			if (!Values.TryGetValue(parentNode, out var source) && Context.ExceptionHandler != null)
+				return;
+
 			XmlName propertyName;
 
 			if (TryGetPropertyName(node, parentNode, out propertyName)) {
@@ -95,7 +97,8 @@ namespace Xamarin.Forms.Xaml
 				parentElement = parentNode as IElementNode;
 			}
 
-			var value = Values[node];
+			if (!Values.TryGetValue(node, out var value) && Context.ExceptionHandler != null)
+				return;
 
 			if (propertyName != XmlName.Empty || TryGetPropertyName(node, parentNode, out propertyName)) {
 				if (Skips.Contains(propertyName))
@@ -103,12 +106,14 @@ namespace Xamarin.Forms.Xaml
 				if (parentElement.SkipProperties.Contains(propertyName))
 					return;
 
-				var source = Values[parentNode];
+				if (!Values.TryGetValue(parentNode, out var source) && Context.ExceptionHandler != null)
+					return;
 				ProvideValue(ref value, node, source, propertyName);
 				SetPropertyValue(source, propertyName, value, Context.RootElement, node, Context, node);
 			}
 			else if (IsCollectionItem(node, parentNode) && parentNode is IElementNode) {
-				var source = Values[parentNode];
+				if (!Values.TryGetValue(parentNode, out var source) && Context.ExceptionHandler != null)
+					return;
 				ProvideValue(ref value, node, source, XmlName.Empty);
 				string contentProperty;
 				Exception xpe = null;
@@ -143,7 +148,8 @@ namespace Xamarin.Forms.Xaml
 					throw xpe;
 			}
 			else if (IsCollectionItem(node, parentNode) && parentNode is ListNode) {
-				var source = Values[parentNode.Parent];
+				if (!Values.TryGetValue(parentNode.Parent, out var source) && Context.ExceptionHandler != null)
+					return;
 				ProvideValue(ref value, node, source, XmlName.Empty);
 				var parentList = (ListNode)parentNode;
 				if (Skips.Contains(parentList.XmlName))

Xamarin.Forms.Xaml/CreateValuesVisitor.cs

@@ -43,12 +43,15 @@ namespace Xamarin.Forms.Xaml
 		{
 			object value = null;
 
-			XamlParseException xpe;
 			var type = XamlParser.GetElementType(node.XmlType, node, Context.RootElement?.GetType().GetTypeInfo().Assembly,
-				out xpe);
-			if (xpe != null)
+				out XamlParseException xpe);
+			if (xpe != null) {
+				if (Context.ExceptionHandler != null) {
+					Context.ExceptionHandler(xpe);
+					return;
+				}
 				throw xpe;
-
+			}
 			Context.Types[node] = type;
 			if (IsXaml2009LanguagePrimitive(node))
 				value = CreateLanguagePrimitive(type, node);

Xamarin.Forms.Xaml/FillResourceDictionariesVisitor.cs

@@ -25,7 +25,7 @@ namespace Xamarin.Forms.Xaml
 
 		public void Visit(ValueNode node, INode parentNode)
 		{
-			if (!typeof(ResourceDictionary).IsAssignableFrom(Context.Types[((IElementNode)parentNode)]))
+			if (!Context.Types.TryGetValue((IElementNode)parentNode, out var type) || !typeof(ResourceDictionary).IsAssignableFrom(type))
 				return;
 
 			node.Accept(new ApplyPropertiesVisitor(Context, stopOnResourceDictionary: false), parentNode);
@@ -37,10 +37,11 @@ namespace Xamarin.Forms.Xaml
 
 		public void Visit(ElementNode node, INode parentNode)
 		{
-			var value = Values[node];
-			XmlName propertyName;
+			if (!Values.TryGetValue(node, out var value) && Context.ExceptionHandler != null)
+				return;
+			
 			//Set RD to VE
-			if (typeof(ResourceDictionary).IsAssignableFrom(Context.Types[node]) && ApplyPropertiesVisitor.TryGetPropertyName(node, parentNode, out propertyName)) {
+			if (typeof(ResourceDictionary).IsAssignableFrom(Context.Types[node]) && ApplyPropertiesVisitor.TryGetPropertyName(node, parentNode, out XmlName propertyName)) {
 				if ((propertyName.LocalName == "Resources" ||
 					 propertyName.LocalName.EndsWith(".Resources", StringComparison.Ordinal)) && value is ResourceDictionary) {
 					var source = Values[parentNode];
@@ -51,7 +52,8 @@ namespace Xamarin.Forms.Xaml
 
 			//Only proceed further if the node is a keyless RD
 			if (   parentNode is IElementNode
-				&& typeof(ResourceDictionary).IsAssignableFrom(Context.Types[((IElementNode)parentNode)])
+				&& Context.Types.TryGetValue((IElementNode)parentNode, out var parentType)
+				&& typeof(ResourceDictionary).IsAssignableFrom(parentType)
 				&& !((IElementNode)parentNode).Properties.ContainsKey(XmlName.xKey))
 				node.Accept(new ApplyPropertiesVisitor(Context, stopOnResourceDictionary: false), parentNode);
 			else if (   parentNode is ListNode
@@ -74,7 +76,8 @@ namespace Xamarin.Forms.Xaml
 			if (enode is null)
 				return false;
 			if (   parentNode is IElementNode
-			    && typeof(ResourceDictionary).IsAssignableFrom(Context.Types[((IElementNode)parentNode)])
+				&& Context.Types.TryGetValue((IElementNode)parentNode, out var parentType)
+				&& typeof(ResourceDictionary).IsAssignableFrom(parentType)
 			    && !((IElementNode)parentNode).Properties.ContainsKey(XmlName.xKey))
 				return true;
 			if (   parentNode is ListNode