I have same issue over and over again on iOS.
GeoView.CollapseAttribution ()
GeoView.TouchesBegan (Foundation.NSSet touches, UIKit.UIEvent evt)
(wrapper managed-to-native) UIKit.UIApplication.xamarin_UIApplicationMain(int,intptr,intptr,intptr,intptr&)
UIApplication.UIApplicationMain (System.Int32 argc, System.String[] argv, System.IntPtr principalClassName, System.IntPtr delegateClassName)
UIApplication.Main (System.String[] args, System.Type principalClass, System.Type delegateClass)
Application.Main (System.String[] args)
Currently I use Esri.ArcGISRuntime.Xamarin.Forms 100.15.2 on 16.4.1 and lower.
the exception is
GeoView.CollapseAttribution () SIGABRT: Object reference not set to an instance of an object.
I have no idea how to solve it.