Called at the exact moment the widget is allocated in memory.
It uses an internal "callable" type, to avoid any @overrides in subclases.
This method should be internal and is required to define the
lifetime cycle of the subclass.
Called before onDelete method. onClose might be used to
dispose resources used by the controller. Like closing events,
or streams before the controller is destroyed.
Or dispose objects that can potentially create some memory leaks,
like TextEditingControllers, AnimationControllers.
Might be useful as well to persist some data on disk.
