Xamarin forms:使用ZXing.Net.Mobile进行扫描时使用System.NullReferenceException？

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:zxing="clr-namespace:ZXing.Net.Mobile.Forms;assembly=ZXing.Net.Mobile.Forms"
             x:Class="YourNamespace.YourPage">
    <zxing:ZXingScannerView x:Name="scannerView" OnScanResult="HandleScanResult" />
</ContentPage>

public partial class YourPage : ContentPage
{
    public YourPage()
    {
        InitializeComponent();
    }

    private void HandleScanResult(Result result)
    {
        // 处理扫描结果
    }
}

<uses-permission android:name="android.permission.CAMERA" />

<key>NSCameraUsageDescription</key>
<string>This app requires access to the camera.</string>

protected override void OnAppearing()
{
    base.OnAppearing();
    scannerView.IsScanning = true;
}

protected override void OnDisappearing()
{
    base.OnDisappearing();
    scannerView.IsScanning = false;
}

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:zxing="clr-namespace:ZXing.Net.Mobile.Forms;assembly=ZXing.Net.Mobile.Forms"
             x:Class="YourNamespace.YourPage">
    <zxing:ZXingScannerView x:Name="scannerView" OnScanResult="HandleScanResult" />
</ContentPage>

using Xamarin.Forms;
using ZXing;
using ZXing.Mobile;

namespace YourNamespace
{
    public partial class YourPage : ContentPage
    {
        public YourPage()
        {
            InitializeComponent();
        }

        private void HandleScanResult(Result result)
        {
            Device.BeginInvokeOnMainThread(() =>
            {
                // 处理扫描结果
                DisplayAlert("Scanned Code", result.Text, "OK");
                scannerView.IsScanning = false;
            });
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();
            scannerView.IsScanning = true;
        }

        protected override void OnDisappearing()
        {
            base.OnDisappearing();
            scannerView.IsScanning = false;
        }
    }
}