toJson method Null safety

Implementation

Map<String, dynamic> toJson() {
  final Map<String, dynamic> data = <String, dynamic>{};
  data['3xs'] = _d3xs;
  data['2xs'] = _d2xs;
  data['xs'] = _xs;
  data['sm'] = _sm;
  data['md'] = _md;
  data['lg'] = _lg;
  data['xl'] = _xl;
  data['2xl'] = _d2xl;
  data['container'] = _container;
  data['spaces'] = _spaces;
  return data;
}