Dealing With A Difficult Topic
It’s reasonable to be concerned about unexpected questions, but those aren’t the most difficult topics or questions. You probably already know the most difficult topics that would come up at your viva.
What don’t you want to talk about? What do you struggle to remember? What might be controversial in your research?
Difficult topics are the kinds of things to take your time with in preparation and in the viva. There’s time. There’s no need to rush but if a difficult topic comes up at the viva there is a need to talk about it.
If you know a difficult topic ahead of your viva then prepare. Ask for help, ask your supervisor, rehearse by discussing the area and taking questions. Don’t try to ignore it. Don’t simply hope that it won’t come up.