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Choosing a random page/round to determine the payment

#1 by Sarah

I want to choose one random round among 2 rounds to pay the participants accordingly. I tried with the provided code in the featured projects. Here is the code I used. 

class Group(BaseGroup):
    app_to_pay = models.StringField()
class Results(Page):
    def before_next_page(group: Group, timeout_happened):
        import random

        participant = group.participant

        # print('participant.app_payoffs is', participant.app_payoffs)

        apps = [
            'round 1',
            'round 2'

        app_to_pay = random.choice(apps)

        group.app_to_pay = app_to_pay

The problem is that I face an error: "group.app_to_pay is None. Accessing a null field is generally considered an error. Or, if this is intentional, use field_maybe_none()" 

If I put initial in the app_to_pay=models.StringField(initial=''), then the error is gone but it shows nothing in the screen. Do you know how I can fix the error? 

Best regards,

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