onCancel property Null safety

1. sendbird_sdk
2. async_operation
3. AsyncTask<T>
4. onCancel property

AsyncTask class

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