toMap method Null safety

Implementation

@override
Map<String, Object?> toMap() {
  return {
    'id': this.id,
    'center': this.center.toMap(),
    'radius': this.radius,
    'color': this.color.value.toRadixString(16),
    'zIndex': this.zIndex,
    'visible': this.visible
  };
}