toJson method Null safety

Implementation

Map<String, dynamic> toJson() {
  return {
    "0": dp0,
    "5": dp5,
    "10": dp10,
    "20": dp20,
    "25": dp25,
    "30": dp30,
    "40": dp40,
    "50": dp50,
    "60": dp60,
    "70": dp70,
    "75": dp75,
    "80": dp80,
    "90": dp90,
    "95": dp95,
    "100": dp100,
  };
}