如何使最终的本地变量不可废？

Question

下面的代码产生错误字符串？ 即使我添加了该行以确保PERF不为null，也无法分配给参数类型“字符串”

Some function() {
  try{
   ...
  final userData;
  userData = json.encode({'token': _token, 'userId': user_Id, "expiryDate": _expiryToken });
  prefs.setString("userData", userData);
} catch (error) {
  throw error;
}


}

    Future<bool>tryAutoLogin() async{
        final prefs = await SharedPreferences.getInstance();
        if (!prefs.containsKey('userData')){
          return false;
        }
        final extractedUserData;
        if (prefs != null){
      extractedUserData = json.decode(prefs.getString("userData")) as Map<String, Object>;
      final expiryDate = DateTime.parse(extractedUserData['expiryDate']);
      if(expiryDate.isAfter(DateTime.now())){
        return false;
      }
      _token = extractedUserData['token'] as String?;
      user_Id = extractedUserData['userId'] as String?;
      _expiryToken = extractedUserData['expiryDate'] as DateTime?;
      return true;
      }
      return false;
      }

行JSON解码会产生错误。问题是什么？