``` Error: A value of type 'num' can't be returned from a function with return type 'int?'. ``` when implementing https://api.flutter.dev/flutter/widgets/ChildIndexGetter.html where the return expression is `(+ x y)` with dynamic x and y.
when implementing https://api.flutter.dev/flutter/widgets/ChildIndexGetter.html where the return expression is
(+ x y)with dynamic x and y.