Fix Exception in XamlDesigner

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.XamlDom/Project/XamlProperty.cs

@@ -248,8 +248,9 @@ namespace ICSharpCode.WpfDesign.XamlDom
 				try {
 					ValueOnInstance = PropertyValue.GetValueFor(propertyInfo);
 					
-					if (this.parentObject.XamlSetTypeConverter != null)
+					if (this.parentObject.XamlSetTypeConverter != null && propertyValue is XamlTextValue) {
 						this.ParentObject.XamlSetTypeConverter(this.parentObject.Instance, new XamlSetTypeConverterEventArgs(this.SystemXamlMemberForProperty, null, ((XamlTextValue) propertyValue).Text, this.parentObject.OwnerDocument.GetTypeDescriptorContext(this.parentObject), null));
+					}
 
 					if (propertyInfo.DependencyProperty == DesignTimeProperties.DesignWidthProperty) {
 						var widthProperty = this.ParentObject.Properties.FirstOrDefault(x => x.DependencyProperty == FrameworkElement.WidthProperty);