尝试;

FirebaseAuth auth = FirebaseAuth.instance;
final User user = await auth.currentUser;
String uid = user!.uid;
await FirebaseFirestore.instance.collection('data').doc(uid).collection('data').doc();

User示例可以是null，使用空Assert运算符（user!.uid）进行显式转换不是一种好的做法，因为它可能会导致运行时异常。
所以试试这个：

FirebaseAuth auth = FirebaseAuth.instance;

final User? user = await auth.currentUser();
final String? uid = user?.uid;

if(user != null && uid != null) {
  // We have already tested 'uid' nullability, so now it is safe to use 'uid!'.
  await FirebaseFirestore.instance
          .collection('data')
          .doc(uid!)
          .collection('data')
          .doc();
} else {
  throw Exception('current user is null.');
}