.NET 8 MAUI Android迁移后HttpClient调用API遇NullReferenceException求助

异常信息 #

异常类型：System.NullReferenceException
错误消息：The current Activity can not be detected. Ensure that you have called Init in your Activity or Application class.

触发场景 #

调用以下HttpClient代码请求API时出现该异常：

var content = new StringContent(jsonData, Encoding.UTF8, "application/json");
var response = await _client.PostAsync(uri, content);

运行环境 #

• .NET 8
• Android
• MAUI
• Visual Studio

背景情况 #

将基于.NET Framework的Android Xamarin应用升级至MAUI

相关代码 #

MainActivity类代码 #

[Activity(Name = "Test.MainActivity", Label = "Test Mobile", Icon = "@drawable/icon"
    , LaunchMode = LaunchMode.SingleTask, MainLauncher = true, ScreenOrientation = ScreenOrientation.Portrait, Theme = "@style/Maui.SplashTheme"
    , ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Microsoft.Maui.MauiAppCompatActivity
{
    internal static MainActivity Instance { get; private set; }

    // for FCM push notifications
    internal static readonly string CHANNEL_ID = "fcm_notification_channel";
    internal static readonly int NOTIFICATION_ID = 100;

    enum EnvCheckType { Root, Debug, Emulator, Hooks, CustomFirmware, IntegrityCheck, WirelessSecurity };
    static BitArray envChecks = new BitArray(7);

    private MyBroadcastReceiver mMessageReceiver = new MyBroadcastReceiver();

    protected override void OnCreate(Bundle savedInstanceState)
    {
        try
        {
            Instance = this;

            base.OnCreate(savedInstanceState);
        }
        catch (Exception ex)
        {
            ExceptionLogger.LogErrorAsync(ex);
        }
    }
}

MainApplication类代码 #

[Application]
public class MainApplication : MauiApplication
{
    // for FCM push notifications
    internal static readonly string CHANNEL_ID = "fcm_notification_channel";
    internal static readonly int NOTIFICATION_ID = 100;

    enum EnvCheckType { Root, Debug, Emulator, Hooks, CustomFirmware, IntegrityCheck, WirelessSecurity };
    static BitArray envChecks = new BitArray(7);

    public MainApplication(nint handle, JniHandleOwnership ownership) : base(handle, ownership)
    {
    }

    protected override MauiApp CreateMauiApp()
    {
        var builder = MauiApp.CreateBuilder();
        builder.UseMauiApp<App>().UseMauiCommunityToolkit();
        
        return builder.Build();
    }
}

已尝试操作 #

调整过MainActivity和HttpClient相关代码，但问题仍未解决，恳请各位提供解决建议。

内容的提问来源于stack exchange，提问作者Chandramouli