toMap method Null safety

Implementation

Map<String, dynamic> toMap() {
  return <String, dynamic>{
    'visibleInPreview': visibleInPreview,
    'positionInLandscapeMode':
        positionInLandscapeMode?.toMap() ?? Watermark.defaultMap(),
    'positionInPortraitMode':
        positionInPortraitMode?.toMap() ?? Watermark.defaultMap(),
  };
}