Just wanted to give you some tricks to prevent you facing the same problems while targeting iOS 13 and Dark Mode:
You certainly read @DavidOrtinau's article https://devblogs.microsoft.com/xamarin/modernizing-ios-apps-dark-mode-xamarin/ and checked out the associated Xappy app on GitHub: https://github.com/davidortinau/Xappy/tree/feature/dark13
The code is not compatible with iOS prior to v13.0. Please check https://github.com/davidortinau/Xappy/issues/47 and bug fix there.
When preparing assets using the new Light/Dark tab:
You still must reference your "Light" assets into tab "Any appearance", otherwise iOS prior to v13.0 won't see any available assets: it will not automatically fall back to "Light" category.