A validation error or warning should be shown when:
- the widget type configuration shows a property using a resource editor
- this property value is not an EL expression
- the referenced resource (directory, template...) is not found on the Studio project
Also, as there is no widget type registry, the "generic" widget type should issue a warning instead of an error when the type is unknown (so that users can use a custom widget that is not known to Studio without getting an error).