Displaying a UIDocumentMenuViewController

I'm trying to display a UIDocumentMenuViewController from the view model in a Xamarin.Forms application. The UIDocumentMenuViewController displays correctly, but when I select a provider the UIDocumentPickerViewController displays a black screen. The view hierarchy looks the same as for the Xamarin DocPicker sample. Am I using the right view controller here?

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;

using UIKit;
using Foundation;
using MobileCoreServices;

using Oban.Mobile.Services;

namespace Oban.Mobile.iOS.Services
{
    public class FilePicker : IFilePicker
    {
        public async Task<ImportedFile> ImportFile(List<string> contentTypes)
        {
            var ubiq = await Task.Run(() => NSFileManager.DefaultManager.GetUrlForUbiquityContainer(null));
            if (ubiq == null)
            {
                throw new Exception("iCloud not available");
            }

            TaskCompletionSource<ImportedFile> tcs = new TaskCompletionSource<ImportedFile>();

            var allowedUTIs = new string[] { UTType.UTF8PlainText,
                    UTType.PlainText,
                    UTType.RTF,
                    UTType.PNG,
                    UTType.Text,
                    UTType.PDF,
                    UTType.Image };

            var activeController = UIControllerHelper.FindActiveViewController();

            UIDocumentMenuViewController pickerMenu = new UIDocumentMenuViewController(allowedUTIs, UIDocumentPickerMode.Import);
            pickerMenu.WasCancelled += (sender, args) => tcs.SetResult(null);
            pickerMenu.DidPickDocumentPicker += (sender, args) =>
            {
                args.DocumentPicker.WasCancelled += (docSender, docArgs) => tcs.SetResult(null);
                args.DocumentPicker.DidPickDocument += (docSender, docArgs) =>
                {
                   ImportedFile file = null;

                    try
                    {
                        var securityEnabled = docArgs.Url.StartAccessingSecurityScopedResource();
                        var data = NSData.FromUrl(docArgs.Url); 

                        file = new ImportedFile(docArgs.Url.LastPathComponent, data.ToArray());
                        tcs.SetResult(file);
                    }
                    catch (Exception excp)
                    {
                        tcs.SetException(excp);
                    }
                    finally
                    {
                        docArgs.Url.StopAccessingSecurityScopedResource();
                    }

                };

                args.DocumentPicker.View.TranslatesAutoresizingMaskIntoConstraints = false;
                args.DocumentPicker.View.BackgroundColor = UIColor.White;

                args.DocumentPicker.ModalPresentationStyle = UIModalPresentationStyle.FullScreen;
                args.DocumentPicker.View.Frame = activeController.View.Frame;
                args.DocumentPicker.View.Bounds = activeController.View.Bounds;
                activeController.PresentViewController(args.DocumentPicker, true, null);
            };


            pickerMenu.ModalPresentationStyle = UIModalPresentationStyle.Popover;
            pickerMenu.View.TranslatesAutoresizingMaskIntoConstraints = false;
            activeController.PresentViewController(pickerMenu, true, null);

            UIPopoverPresentationController presentationPopover = pickerMenu.PopoverPresentationController;
            if (presentationPopover != null)
            {
                presentationPopover.SourceView = activeController.View;
                presentationPopover.PermittedArrowDirections = UIPopoverArrowDirection.Down;
                presentationPopover.SourceRect = activeController.View.Frame;
            }

            return await tcs.Task;
        }
    }

    public static class UIControllerHelper
    {
        public static UIViewController FindActiveViewController()
        {
            UIViewController vc = UIApplication.SharedApplication.KeyWindow.RootViewController;
            while (vc.PresentedViewController != null)
            {
                vc = vc.PresentedViewController;
            }
            return vc;
        }

     }

    public class ImportedFile
    {
        public string FileName { get; private set; }

        public byte[] Content { get; private set; }

        public ImportedFile(string fileName, byte[] content)
        {
            this.FileName = fileName;
            this.Content = content;
        }
    }

    public interface IFilePicker
    {
        Task<ImportedFile> ImportFile(List<string> contentTypes);
    }
}