fix error (dynamic) => dynamic is not a subtype of type 'Widget'

[earthfail]

caused by returning function v (first-screen,second-screen) instead of the (v ctx). Since routes paramter expects Map<String, WidgetBuilder>?, the values of routes map should be functions of BuildContext to Widget. Instead the previous implementation had functions of BuildContext that return functions of BuildContext to Widget.

[dupuchba]

@earthfail I fixed it using slightly different code. Thanks a lot for your report, it's really appreciated