onCancel property Null safety

1. sendbird_sdk
2. async_operation
3. AsyncTask2<T, D>
4. onCancel property

AsyncTask2 class

1. Constructors
2. AsyncTask2
3. Properties
4. arg
5. arg2
6. func
7. hashCode
8. onCancel
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==