You have two question groups, each of them containing questions and answers. You are allowed by LimeSurvey to preview the question/question group.

If you add a new question group (that contains no questions), theoretically you should still be allowed to preview the first two question groups and their questions. However, this is not happening. I get the following error when I want to preview one of the two question groups (or one of their questions) that were working prior to the new question group addition:

This survey cannot be tested or completed for the following reason(s):There are empty question groups in this survey - please create at least one question within a question group.For further information please contact Your Name (email)