Called when the widget is created to generate the initial list of Route objects if initialRoute is not null.
Defaults to defaultGenerateInitialRoutes.
The NavigatorState and initialRoute will be passed to the callback. The callback must return a list of Route objects with which the history will be primed.
When parsing the initialRoute, if there's any chance that the it may
contain complex characters, it's best to use the
characters API. This will ensure
that extended grapheme clusters and surrogate pairs are treated as single
characters by the code, the same way that they appear to the user. For
example, the string "👨👩👦" appears to the user as a single
string.characters.length intuitively returns 1. On the
string.length returns 8, and
final RouteListFactory onGenerateInitialRoutes;