You can’t expect your thesis to be perfect. Your examiners don’t expect it to be perfect. There might be typos or clumsy writing, something vague, but nothing too awful.
But what if there’s a flaw? What if there’s something which is just wrong? A hole in your logic perhaps, or maybe a result or reference that doesn’t support your argument. What then?
First, read your thesis carefully before the viva. Then you can be sure of what’s in there. If you find a flaw you can figure it out. You’ll know how to respond to comments about it in the viva. If you don’t find anything, then you’re probably fine.
If your examiners find something you didn’t, then talk it through with them. Ask questions to get as much information as possible. Think it through. Talk through each point in turn, make notes and see where your discussions lead you.
A flaw can be a tiny imperfection, not a big deal at all. No thesis is perfect. Not every problem is a problem.