HandleCharOrNullResult
Handles a Route.Result.Data matching a Char or null Requires that the Route.Result.Data.bundle is described by a SingleValueNavigationSpec matching NavigationBundleSpecType.CharType either directly or wrapped by NavigationBundleSpecType.OptionalType
Parameters
retain
If true the result will be retained in the NavBackStackEntry. It will be deleted otherwise.
onResult
Method for handling the received Char ornull