C# UIViewcontroller实例获取null&；不调用文件进行查看

我正在生成一个PDF文件，在UIViewcontroller的帮助下，我调用PDF文件进行查看，但由于未知原因，我的UIViewcontroller变为null

    string path = 
    Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
    string filePath = Path.Combine(path,filename);

    //Create a file and write the stream into it.
    FileStream fileStream = File.Open(filePath, FileMode.Create);
    stream.Position = 0;
    stream.CopyTo(fileStream);
    fileStream.Flush();
    fileStream.Close();

    //Invoke the saved document for viewing
    UIViewController currentController = 
    UIApplication.SharedApplication.KeyWindow.RootViewController;
    while (currentController.PresentedViewController != null)
        currentController = currentController.PresentedViewController;
    UIView currentView = currentController.View;

    QLPreviewController qlPreview = new QLPreviewController();
    QLPreviewItem item = new QLPreviewItemBundle(filename, filePath);
    qlPreview.DataSource = new PreviewControllerDS(item);

    currentController.PresentViewController(qlPreview, true, null);

这是我的AppDelegate.cs文件请检查

 [Register("AppDelegate")]
public partial class AppDelegate : 
global::Xamarin.Forms.Platform.iOS.FormsApplicationDelegate
{



    //
    // This method is invoked when the application has loaded and is ready to run. In this 
    // method you should instantiate the window, load the UI into it and then make the window
    // visible.
    //
    // You have 17 seconds to return from this method, or iOS will terminate your application.
    //
    public override bool FinishedLaunching(UIApplication app, NSDictionary options)
    {
        global::Xamarin.Forms.Forms.Init();
        //CarouselViewRenderer.Init();

        FFImageLoading.Forms.Platform.CachedImageRenderer.Init();

        LoadApplication(new App());
        UIApplication.SharedApplication.StatusBarHidden = true;


        return base.FinishedLaunching(app, options);
    }
}

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参考以下代码（我通过DependencyService实现它）

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注意：检查pdf的文件路径是否正确。

参考以下代码（我通过DependencyService实现）

---

注意：检查您的pdf文件路径是否正确。

您在AppDelegate.cs中设置了rootviewcontroller吗？@LucasZhang MSFT再次检查我的问题我已编辑代码您在AppDelegate.cs中设置了rootviewcontroller吗？@LucasZhang MSFT再次检查我的问题我已编辑代码它引发了类似这样的异常“您正在调用一个只能从ui线程调用的uikit方法”相同的代码和平也在其他按钮上执行，但该按钮仅查看我的pdf，但在这里它不起作用，但此代码先写入文件，然后再查看。感谢我们得到了解决方案。问题出在我们的代码中。它引发了类似这样的异常”您正在调用一个只能从ui线程调用的uikit方法“同样的代码和平也在其他按钮上执行，但该按钮仅查看我的pdf，但在这里它不起作用，但此代码先写入文件，然后再查看。感谢我们得到了解决方案。问题出在我们的代码中。

[assembly: Dependency(typeof(OpenPdfFile))]
namespace xxx.iOS
{
    public class OpenPdfFile : IGetRootViewController
    {


        public void GetRootViewController()
        {


            string path  = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
            string filePath = Path.Combine(path,filename);

            //Create a file and write the stream into it.
            FileStream fileStream = File.Open(filePath, FileMode.Create);
            stream.Position = 0;
            stream.CopyTo(fileStream);
            fileStream.Flush();
            fileStream.Close();

            var previewController = new QLPreviewController();
            previewController.DataSource = new QuickLookSource("file://"+filePath);

            UIViewController currentController =
    UIApplication.SharedApplication.KeyWindow.RootViewController;
            while (currentController.PresentedViewController != null)
                currentController = currentController.PresentedViewController;

            currentController.PresentViewController(previewController,true,null);
        }




    }


    public class QuickLookSource : QLPreviewControllerDataSource
    {
        string filePath;

        public QuickLookSource(string path)
        {
            filePath = path;
        }

        public override nint PreviewItemCount(QLPreviewController controller)
        {
            return 1;
        }

        public override IQLPreviewItem GetPreviewItem(QLPreviewController controller, nint index)
        {
            return new PreviewItem(filePath);
        }
    }

    public class PreviewItem : QLPreviewItem
    {
        NSUrl fileUrl;

        public override NSUrl ItemUrl => fileUrl;
        public override string ItemTitle => fileUrl.LastPathComponent;

        public PreviewItem(string url)
        {
            fileUrl = new NSUrl(url,true);
        }
    }


}