useFirestoreInfiniteQuery fetchNextPage() causes error.

The error:

Unhandled Runtime Error
FirebaseError: Function startAfter() called with invalid data. Unsupported field value: undefined

I get this error, when I click on the Load More button twice, my collection has no more documents, but seems like the button is not disabled and lastDocument is undefined which gets passed to startAfter().

```
const posts = useFirestoreInfiniteQuery("posts", postsQuery, (snapshot) => {
const lastDocument = snapshot.docs[snapshot.docs.length - 1];

// Get the next 20 documents starting after the last document fetched.
return query(postsQuery, startAfter(lastDocument));
});
```

fetchNextPage() causes this error.

```
const posts = useFirestoreInfiniteQuery(...);

posts.fetchNextPage()}
>
Load More

```

What is the reason for this bug?