A little bravery goes a long way. Yes, the unexpected question could scare you, or an honest mistake that seems huge, but you have talent, you have knowledge, you have your thesis.
This should lead to a confidence that you must be good. Good enough to meet with your examiners and defend your work. The tiniest amount of courage will see you through. The smallest reserve in the face of worry or fear. You can find it.
It’s normal to be nervous in anticipation of an important event like the viva. Find your courage. Be brave.
If you could get to submission, you can get through the viva.