The persistent bottom sheet to display.
A persistent bottom sheet shows information that supplements the primary content of the app. A persistent bottom sheet remains visible even when the user interacts with other parts of the app.
A closely related widget is a modal bottom sheet, which is an alternative to a menu or a dialog and prevents the user from interacting with the rest of the app. Modal bottom sheets can be created and displayed with the showModalBottomSheet function.
The value of bottomSheet can be any widget at all. It's unlikely to actually be a BottomSheet, which is used by the implementations of showBottomSheet and showModalBottomSheet. Typically it's a widget that includes Material.
final Widget? bottomSheet;