Error testing Xamarin.Forms app in iOS

Hello, I developed a Xamarin.Forms app which works fine in Android. Now I want to test it in iOS.
In my Mac, I have VS2017 and load the solution. It compiles OK and it runs OK in an iPhone emulator... but in certain part of the app I get this exception : http://prntscr.com/h4xa4m how can I debug it?

Thanks