Vaadin 8 used to have the ComboBox.setNullSelectionAllowed(true) function which enabled the user to select an empty item in the ComboBox. Vaadin 14 does not seem to have the same function, so what to do next?

At first, I tried to add the null item amongst the list of items set via setItems():

combobox.setItems([null] + items)

But that only resulted in a NullPointerException with the message "Cannot provide an id for a null item." thrown from DataProvider.getId(). You could hack around, by overriding the getId() method and making it return an empty string in case of a null item, but once you start using lazy loading, you’ll quickly hit a dead end.

The solution is to show the “Clear” button as follows:

combobox.setClearButtonVisible(true)

Note: for all Vaadin 8->14 migration tips see the karibu-migration project.