This is not yet supported. But will be soon in Errai 2.1. Although, Instance<?> does not exactly work like that.
Instead of doing what you're trying to do, you can simply use the Bean Manager: https://docs.jboss.org/author/display/ERRAI/Client-Side+Bean+Manager
Collection<IOCBeanDef> bean = manager.lookupBeans(Activity.class);
// check to see if the bean exists
(Activity activityBean : beans
// get the instance of the bean
Activity bean = (Activity) activityBean.getInstance();