If a validator is attached to the final card (submit card)  no validation events are triggered.

Validation is never fired on the final 'submit' card. Since _abstractIncrementStep does sufficient card index bounds checks (ie we never go past our last card) -- validating on _onNextClick while 'submitable' works. 

The if/else clause should be refactored to include this new validation step and just return void instead of relying on _abstractIncrementStep's error checking.