Wizard step validator

Hi.

I defined afterVK validator for my wizard step (extends DefaultUIComponentValidator).

I have 2 questions i cann't find answers for in customization guide.

1) Validator works in both ways (Back and Next). I need it to work only when moving forward. How to disable it for "Back" button?

2) If i have an error while validation process, i show it at pop-up window by specifying UIValidationFeedbackMsg for UIValidationResult. How should i define UIValidationResult to disable pop-up if a i have no error. I have no error - i don't want show anything for user. But now pop-up window with NULL message arises.

Thanks.