App.Current.Resources - availability?

[TopperDEL]

I have a Styles.xaml that contains a Converter that uses a SolidColorBrush from the App.Current.Resources that is also defined in the same Styles.xaml. The Converter reads the color once in its constructor by calling App.Current.Resources["MyColor"]. That works on Windows but on Mac it is null.
If I later do the same call, the resource is available. So the workaround here is to simply check for null and lazy-load the resource later. But I'm afraid this "leaks" into more places in my codebase. So something expects a resource to be there but it is not loaded/initialized yet.

First question: is there a more robust way of loading resources in this case?
Second question: should this be an issue as the behavious differs between Windows and Mac/Desktop?