I’m having the abstractMethodError as well. Just updated gson from 2.2.4 to 2.3.x and it still is failing. I had opened a ticket in the addon forum on Friday asking for help on this - I listed a stack trace there… Thanks - nbc
vaadin changed the implantation of JavaScriptFunction so it wont work for newer vaadin versions. use 7.3.3 till 7.4+ library comes. have to change the whole implementation.