我试图返回值在用户中，我得到这个错误，如果我试图通过添加null符号修复这个错误，它不会返回任何值，因为所有这些值将为null，我想返回这个值并保存它，这样我就可以使用它，什么可以是解决方案，请指导

FirebaseAuth auth = FirebaseAuth.instance;
User? user = FirebaseAuth.instance.currentUser;
CollectionReference users = FirebaseFirestore.instance.collection('users');

Future<void> addUser(context) {
  //before we add user we need to check this user already signedin earlier using same number or not
  //if signedIn before no need to add , because it may have added already. so just need to skip adduser



  return users
      .add({
    'uid': user.uid,
    'mobile':  user.phoneNumber,
    'email': user.email
  })
      .then((value){
        //after add data to firebase then will go to next screen
    //if you pushreplacementName or push replacement, then u cant go back to previous screen
    Navigator.pushReplacementNamed(context, LocationScreen.id);

  })
      .catchError((error) => print("Failed to add user: $error"));
}