Xamarin.Android TravelSample InvalidCastException on UserDialogs.Init() in MainActivity.cs

[bdgza]

I am running the sample app in Visual Studio for Mac 8.0.8 with Xcode 10.2.1. The NuGets have not been updated, the Xamarin Forms is 2.5.0.91635. I do not have a backend installed, so looking up a hotel in Hotel Search throws an exception and the alert dialog for "Error fetching hotels" is triggered. The initialisation for Acr.UserDialogs (6.5.1) is done using:
UserDialogs.Init(() => (Activity)ApplicationContext);
in MainActivity.cs. Instead of showing the alert dialog this fails with an InvalidCastException.

[MonoDroid] UNHANDLED EXCEPTION:
[MonoDroid] System.InvalidCastException: Specified cast is not valid.
[MonoDroid] at TravelSample.Droid.MainActivity.b__0_0 () [0x00000] in /Users/bdegroot/Downloads/mobile-travel-sample-master/dotnet/TravelSample/TravelSample.Android/MainActivity.cs:47
[MonoDroid] at Acr.UserDialogs.UserDialogsImpl.Alert (Acr.UserDialogs.AlertConfig config) [0x0000d] in C:\dev\acr\userdialogs\src\Acr.UserDialogs.Android\UserDialogsImpl.cs:34
[MonoDroid] at Acr.UserDialogs.AbstractUserDialogs.AlertAsync (Acr.UserDialogs.AlertConfig config, System.Nullable`1[T] cancelToken) [0x00054] in C:\dev\acr\userdialogs\src\Acr.UserDialogs.Interface\AbstractUserDialogs.cs:130
[MonoDroid] at TravelSample.Core.ViewModels.HotelListViewModel.LookupAsync () [0x00146] in /Users/bdegroot/Downloads/mobile-travel-sample-master/dotnet/TravelSample/TravelSample.Core/ViewModels/HotelListViewModel.cs:268
[MonoDroid] at TravelSample.Core.ViewModels.HotelListViewModel.<get_LookupCommand>b__13_0 () [0x0001f] in /Users/bdegroot/Downloads/mobile-travel-sample-master/dotnet/TravelSample/TravelSample.Core/ViewModels/HotelListViewModel.cs:221
[MonoDroid] at System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.b__6_0 (System.Object state) [0x00000] in <58604b4522f748968296166e317b04b4>:0
[MonoDroid] at Android.App.SyncContext+<>c__DisplayClass2_0.b__0 () [0x00000] in <788a34f7a7b84486905dfde786529d42>:0
[MonoDroid] at Java.Lang.Thread+RunnableImplementor.Run () [0x00008] in <788a34f7a7b84486905dfde786529d42>:0
[MonoDroid] at Java.Lang.IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this) [0x00009] in <788a34f7a7b84486905dfde786529d42>:0
[MonoDroid] at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.34(intptr,intptr)

Changing the line to:
UserDialogs.Init(this);
fixes the problem (in this case).