routes property Null safety
This tab view's routing table.
When a named route is pushed with Navigator.pushNamed inside this tab view, the route name is looked up in this map. If the name is present, the associated widgets.WidgetBuilder is used to construct a CupertinoPageRoute that performs an appropriate transition to the new route.
If the tab view only has one page, then you can specify it using builder instead.
This routing table is not shared with any routing tables of ancestor or descendant Navigators.
final Map<String, WidgetBuilder>? routes;