Accessing user-related data with "auth.currentUser?.uid" error

[EhanDuan]

Hi, I am using Nextjs and firebase to build an webapp.

Expected

When a user requests the page, It will show some data about this user.

Current Way

• What I did: using getServerSideProps to fetch data. It is from NextJs which will execute when the page is requested.
• Here I get userIdfirst, using firebase auth. Then, I use userId to access the doc in the "users" collection . After that, I pass the data I want to props.

Question

The problem could be shown in the following images. The auth.currentUser?.uid might return string or null. But I can make sure I have logged in when testing. It should be string result.

But it popped error like below.

However, if I use string literal ID instead of above code. There is no error.

I am assume the error exist in auth.currentUser?.uid. And doc() function somehow cannot assert variable userId could be used for function param.

How do I solve this problem? Thanks for your help!