Minimal failing example, V8 ComboBox not deleting text of previous selected item

vaadin8.groovy

// run with `spring run --watch vaadin.groovy`
@Grapes([
@Grab('com.vaadin:vaadin-spring-boot-starter:2.0.1'),
@Grab('com.vaadin:vaadin-archetype-application:8.0.5'),
])
import com.vaadin.ui.*
import java.time.*

@com.vaadin.spring.annotation.SpringUI
@com.vaadin.annotations.Theme("valo")
class MyUI extends UI {
        protected void init(com.vaadin.server.VaadinRequest request) {
                def cb = new ComboBox()
                cb.items = [1]
                cb.value = 1
                setContent( 
                        new CssLayout(
                                cb, 
                                new Button("change items and set value=null in CB; but visual value in CB remains 1", {
                                        cb.items = [42]
                                        cb.value = null
                                } as Button.ClickListener),
                        )
                )
        }
}