I’m putting in my two weeks notice. When my boss flips and demands to know where I’m going, how can I professionally tell them it’s none of their business?

Basically I have a vindictive boss and they are very toxic. I have done and gone above and beyond for this company and I'm done with the mistreatment and things that have been said to me. I've recently accepted a position at another company that better aligns with my career goals and needless to say, better pay. I expect my boss to flip out when I hand in my two weeks notice. I wouldn't be surprised if I was fired on the spot. If I don't get fired they are going to demand to know where I am going. I have always been professional and want to stay that way. What's the most polite way to tell them it is none of their business? Any tips on how to prepare for the hellish 2 weeks if I don't get fired?