code: def next_question(self):
          
          for key, questions in self.questions.items():
              if key in self.answers:
                  continue
              for question in questions:
                  if self.check_condition(question._condition):
                      return question
          return None
docs: Returns the next `Question` in the questionnaire, or `None` if there
          are no questions left. Returns first question for whose key there is no
          answer and for which condition is satisfied, or for which there is no
          condition.