Looks like version [2.1]
(https://vaadin.com/directory/component/addon-test-helpers/2.1) does not work with Vaadin 8.0+ but needs at least Vaadin 8.2. Use [version 2.0]
(https://vaadin.com/directory/component/addon-test-helpers/2.0) for Vaadin 8.0 to 8.1.