Steps to reproduce

Create a survey, add a group and an optional question.
Go to "theme settings" and choose something other than "Show popups".
Activate and run the survey.
Don't answere the optionnal question and click submit button.
The error appears.

Expected result

When I choose "onepage" I expect a popin appear
When I choose "non" I expect a block in the page who ask to anwsere or to continue whitout answering.

Actual result

The user is blocked or has to answer the question he did not want to answer.