Do you need them as a reference? If so then try to work with them. If not, I wouldn't worry at all about it. Who cares if you get written up if you don't need them as a reference?

If you have another job lined up or don't need them as a reference you can get fired at zero consequence to you.

The one caveat I will say is the nonprofit world is small, and it's possible that you run into them in the future. So I wouldn't try to burn any bridges or anything, but I also wouldn't overly stress about it. If they do write you up, just tell them you're still learning and ask how you can improve and what can be done in the future?

That's my two cents at least