EBADF (Bad file descriptor) with Andoid 10 target with PackageInstaller API

Before I updated to visual studio 19 and Android 10 (Q), I successfully installed third party apps with my app with the following code.

PackageInstaller installer = activity.PackageManager.PackageInstaller;
PackageInstaller.SessionParams sessionParams = new PackageInstaller.SessionParams(PackageInstallMode.FullInstall);
int sessionId = installer.CreateSession(sessionParams);
PackageInstaller.Session session = installer.OpenSession(sessionId);

var input = new FileStream(pfad, FileMode.Open, FileAccess.Read);
var packageInSession = session.OpenWrite("package", 0, -1);
input.CopyTo(packageInSession);
packageInSession.Close();
input.Close();
packageInSession.Dispose();
input.Dispose();

//That this is necessary could be a Xamarin bug.
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();

Intent intent = new Intent(activity, activity.Class);
intent.SetAction("com.example.android.apis.content.SESSION_API_PACKAGE_INSTALLED");
PendingIntent pendingIntent = PendingIntent.GetActivity(activity, 0, intent, 0);
IntentSender statusReceiver = pendingIntent.IntentSender;

// Commit the session (this will start the installation workflow).
session.Commit(statusReceiver);

When i Dispose() the streams, i get an IOException: write failed (EBADF) bad file descriptor which would indicate a bad APK.

But this is unlikely because the code in visual studio 2017 works with the Android 9 target.

Hope somebody can help me and thank you in advance!