PeriodSerializer constructor Null safety

1. wings_data_model
2. wings_data_model
3. PeriodSerializer
4. PeriodSerializer constructor

PeriodSerializer class

1. Constructors
2. PeriodSerializer
3. Properties
4. hashCode
5. runtimeType
6. types
7. wireName
8. Methods
9. deserialize
10. noSuchMethod
11. serialize
12. toString
13. Operators
14. operator ==