onOpen property
Called just before the overlay is displayed.
This function needs to return a bool, either from an async scope
or as a Future.
If this function returns false, this step will be marked complete
and therefore be skipped, i.e. it will not be opened.
In this case, we try to open the next step.
When the Future finishes and evaluates to true, this step will be shown.
Implementation
final Future<bool> Function()? onOpen;