Convert complex Objects to Json || Serialize Objects - json

Question

I need to locally serialize an Object. What is the best way to do this?
I thought the best solution would be to convert my object into a json, the encode this json to a string and serialize this string with shared preferences.
The problem is, there is no toJson() method (as far as I know), that converts the whole object to a json. So I need to create this method myself. However my Object is quite complex with nested Objects and List of Objects and some attributes can be null at times. So a toJson() method would me very complex and cumbersome. It might look something like this just even more complex:
Workout class I want to convert to json:
class Workout {
String id;
String name;
Routine routine;
List<Exercise> exercises;
Map<String, dynamic> toJson() => {
'name': name,
'exercises': [
exercises.map((Exercise exercise) {
print(exercise.toJson());
return exercise.toJson();
}).toString(),
],
};
}
Exercise Class:
class Exercise {
String id;
String presetId;
Timings timings;
String progression;
Preset preset;
Map<String, dynamic> toJson() => {
id != null ? 'id': id : '',
'presetId': presetId,
progression != null ? 'progression': progression : '',
timings == null ? '' : 'timings': timings.toJson(),
};
}
Timings Class:
class Timings {
int holdDown;
int positives;
int hold;
int negatives;
Map<String, dynamic> toJson() => {
'holdDown': holdDown,
'positives': positives,
'hold': hold,
'negatives': negatives
};
}
And the Object might even get bigger and I still need to create functions fromJson(). Also the conditional satements don't work well because there seems to be a conflict between ":" of the statement ":" and the actual json...
Any ideas?