private void BtnLoadXml_Click(object sender, EventArgs e)
{
XmlSerializer xmlSeri = new XmlSerializer(typeof(PKW));
FileStream file = File.Open("xmlFahrzeuge.xml", FileMode.Open);
PKW pkw1 = (PKW)xmlSeri.Deserialize(file);
MessageBox.Show(pkw1.ToString());
file?.Close();
}