encode method Null safety

Implementation

Object encode() {
  final Map<Object?, Object?> pigeonMap = <Object?, Object?>{};
  pigeonMap['topImage'] = topImage;
  pigeonMap['bearingImage'] = bearingImage;
  pigeonMap['shadowImage'] = shadowImage;
  pigeonMap['scaleExpression'] = scaleExpression;
  return pigeonMap;
}