Crash on opening a new page

Hi I'm trying to open a new page (master-detail-pattern) and get a crash I can not debug, because the stacktrace doesn't really help:

        public RelayCommand<SettingsEntry> OpenCommand {
            get {
                return new RelayCommand<SettingsEntry> (async settingsEntry => _navigationService.NavigateTo (NavigationServiceHelper.SettingsDetailPage, new NavigationParameters () {
                    Parameter = settingsEntry
                }));
            }
        }

Do you have an idea how to debug errors like this?

Here parts of the stacktrace:

mscorlib.dll!System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() in /Users/builder/data/lanes/2377/73229919/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/external/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143
mscorlib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.AnonymousMethod__0(System.Runtime.ExceptionServices.ExceptionDispatchInfo state) in /Users/builder/data/lanes/2377/73229919/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/external/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:994
Xamarin.iOS.dll!UIKit.UIKitSynchronizationContext.Post.AnonymousMethod__0() in /Users/builder/data/lanes/2377/73229919/source/maccore/src/UIKit/UIKitSynchronizationContext.cs:24
Xamarin.iOS.dll!Foundation.NSAsyncActionDispatcher.Apply() in /Users/builder/data/lanes/2377/73229919/source/maccore/src/Foundation/NSAction.cs:163
Xamarin.iOS.dll!UIKit.UIApplication.UIApplicationMain() in 
Xamarin.iOS.dll!UIKit.UIApplication.Main(string[] args, System.IntPtr principal, System.IntPtr delegate) in /Users/builder/data/lanes/2377/73229919/source/maccore/src/UIKit/UIApplication.cs:77