toMap method Null safety

Implementation

@override
Map<String, Object?> toMap() {
  return Map.from(super.toMap())
    ..addAll({
      'maxZoom': this.maxZoom,
      'minZoom': this.minZoom,
      'visibleMapBounds': this.visibleMapBounds.toMap(),
      'maxTileTmp': this.maxTileTmp,
      'tileLoadType': this.tileLoadType.index,
      'url': this.url
    });
}