When a Navigator is configured with a RestorationId, and all routes returned by onGenerateInitialRoutes are not restorable, a cryptic error is thrown, see https://github.com/flutter/gallery/pull/389/files#r544521147 and the original comment. We should throw a better error telling developers that the routes returned by onGenerateInitialRoutes should be made restorable by giving them a settings.name that can be looked up in the routes table during restoration.