How to make final local variables non nullable?

Question

The code below produces the error String? cannot be assigned to the parameter type 'String'

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;
      }

The line where there is decoding of json produces the error even after i added the line to make sure prefs is not null. What is the problem?