Fixed NullReferenceExceptions.

src/Markup/Perspex.Markup.Xaml/Converters/PerspexPropertyTypeConverter.cs

@@ -36,7 +36,7 @@ namespace Perspex.Markup.Xaml.Converters
             {
                 var styleType = context.TypeRepository.GetByType(typeof(Style));
                 var style = (Style)context.TopDownValueContext.GetLastInstance(styleType);
-                type = style.Selector.TargetType;
+                type = style.Selector?.TargetType;
 
                 if (type == null)
                 {

src/Markup/Perspex.Markup.Xaml/MarkupExtensions/StyleResourceExtension.cs

@@ -23,7 +23,7 @@ namespace Perspex.Markup.Xaml.MarkupExtensions
             if (styleHost == null)
             {
                 throw new ParseException(
-                    $"StyleResource cannot be assigned to an object of type '{styleHost.GetType()}'.");
+                    $"StyleResource cannot be assigned to an object of type '{extensionContext.TargetObject.GetType()}'.");
             }
 
             // HACK: This should be as simple as: