Others have answered the question about should you give access. As well as the downsides in not doing as asked. Let me give you the perspective of a manager.
Let's say I have a worker with skills I'm really not qualified to properly evaluate. They produce something, maybe it works, maybe it has issues. Maybe I don't even know whether the kind and number of issues seen are "normal" or if it's indicative of bad work. Perhaps it's the opposite and the work is of a really high caliber but I have no clue and my boss is demanding more be done.
As a manager I need to make sure things are actually going as they should.
I can't exactly just take your word because you'll likely just tell me that the quality of your work is pure awesome - and you might have an overly inflated opinion of yourself.
I could hire a set of consultants to evaluate the work. Then again, no matter what the work is like if the consultants believe that by bad mouthing it they will get a beefier contract with me then they'll bad mouth it. So, my trust level going that route is already low.
This means I need to talk with people I trust. I'll have you package up your work and let these people take a look at it. They won't look too far, because it's going to be a personal favor. Hopefully they actually know what they are talking about. Which leaves a few possibilities.
- First, they are experts and think what you've done is at least good enough.
- Second, they are experts and think your work is a disaster.
- Third, they aren't really experts but don't want to get involved further and just say that it's good.
- Fourth, they aren't really experts but don't want to look bad and therefore pick on some rather meaningless things to have you change.
In each of these scenarios the only one you should be concerned about is if the boss actually knows some good programmers who believe your work is garbage. If that's the case then your best bet is to try and get a meeting with them to go over your decisions while listening carefully to what they have to say.
To be fair, the fact you don't want someone else to see it makes me believe you have something to hide. Perhaps you know your work isn't really that great and are afraid of what someone else might think about it. If this is the case then being able to discuss it with a third party would be a very good thing for you. So, I'd suggest you talk to your manager and ask if they could do the code review on site, with you. That way you can both learn from their experience as well as explain the decisions you've made.
Every great programmer I know actually wants people to look over their work. Why? Because coding is hard and even highly experienced people can make basic mistakes. Whether that's in the code itself or even in the overall architecture. We know that if it's caught early enough then the pain for fixing it is minimized. We also know that no one is perfect and we can all learn from each other.