public Task RequestNavigate <TViewModel>(INavigationParameters parameters)
            where TViewModel : ReactiveViewModel
        {
            if (parameters == null)
            {
                throw new ArgumentNullException("parameters", "parameters is null.");
            }

            return(NavigableRegion.RequestNavigate <TViewModel>(parameters));
        }
        public Task RequestNavigate(ReactiveViewModel navigationTarget, INavigationParameters parameters)
        {
            if (navigationTarget == null)
            {
                throw new ArgumentNullException("navigationTarget", "navigationTarget is null.");
            }
            if (parameters == null)
            {
                throw new ArgumentNullException("parameters", "parameters is null.");
            }

            return(NavigableRegion.RequestNavigate(navigationTarget, parameters));
        }