This defines the behavior and location of the snack bar.
Defines where a SnackBar should appear within a Scaffold and how its
location should be adjusted when the scaffold also includes a
FloatingActionButton or a BottomNavigationBar
If this property is null, then SnackBarThemeData.behavior of
ThemeData.snackBarTheme is used. If that is null, then the default is
final SnackBarBehavior? behavior;