how to parse this json to object in flutter?

Question

how to convert this json? rateinfo i splited. i am new for flutter development. i try to convert but error occurred. what changes are needed?

Unhandled Exception: Invalid argument(s) E/flutter ( 3313): #0 _StringBase.+ (dart:core-patch/string_patch.dart:279:43)

        "rateInfo": "{\"base\":{\"quan\":\"5\",\"rate\":\"20\",\"amt\":\"100\"},
        \"fuel\":{\"quan\":\"2\",\"rate\":\"500\",\"amt\":\"1000\"},
        \"unload\":{\"quan\":\"2\",\"rate\":\"200\",\"amt\":\"400\"},
        \"detention\":{\"quan\":\"4\",\"rate\":\"400\",\"amt\":\"1600\"},
        \"misc_char\":{\"title\":[\"Title1\",\"Title2\"],\"quan\":[\"2\",\"3\"],\"rate\":[\"230\",\"100\"],\"amt\":[\"460\",\"300\"]}}"
 

  class Album {
      final int status;
      final String error;
    
      const Album({required this.status, required this.error});
    
      factory Album.fromJson(Map<String, dynamic> json,BuildContext context) {
        int stat=json['status'];
        if(stat==200)
        {
         // Getbase.fromJson(jsonDecode(json['base']));
          print("vallll"+json['base']);
          Getbase.fromJson(json['base']);
        }
        return Album(
          status: json['status'],
          error: json['error'],
    
        );
      }
    
    }
    
    
    
class Getbase {
      String quan;
      String rate;
      String amt;
    
    
      Getbase(this.quan,
          this.rate,
          this.amt,
          );
    
      Getbase.fromJson(Map<String, dynamic> json)
          : quan= json['quan'],
            rate= json['rate'],
            amt= json['amt'];
    
      Map<String, dynamic> toJson() =>
          {
            'quan' : quan,
            'rate': rate,
            'amt': amt,
    
          };
    }