When we leave the page, unsubscribe.

ImageSelectionPage.xaml.cs

/// <summary>
///     Unsubsribe from the MessagingCenter on disappearing.
/// </summary>
protected override void OnDisappearing()
{
    base.OnDisappearing();
    MessagingCenter.Unsubscribe<App, List<string>>((App)Xamarin.Forms.Application.Current, "ImagesSelectedAndroid");
    MessagingCenter.Unsubscribe<App, List<string>>((App)Xamarin.Forms.Application.Current, "ImagesSelectediOS");
    GC.Collect();
}