Question for hiring managers: Should I click No to contacting an old employer for a job I just quit on bad terms?

I just quit a job suddenly by contacting HR and telling them that my boss was hostile and making me miserable. HR was on my side and they said they are going to have serious talks with some people. Now I'm applying to every job that I can, but when an application asks if they can contact my last employer, I'm not sure if I should click yes or no. I don't want prospective employers to call my boss (who would definitely badmouth me), but I also don't want to have a gap showing in my work history. What should I do?

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