AbstractMethodError after fixing compile errors

[Artur-]

1. Use https://github.com/Artur-/spring-boot-loaded-vaadin
2. Remove the listener code in MyUI.java
3. Run using mvn install spring-boot:run
4. Open http://localhost:8080/
5. Add the listener code back but leave imports out, creating a broken inner class
6. Reload the page and click the button - you will get an AbstractMethodError
7. Go back and add the imports
8. Reload the page and click the button - you still get an AbstractMethodError