// fill fuel into selected fuel tank
private void FillTankButton_Click(object sender, RoutedEventArgs e)
{
int amountOfFuel;
if (!string.IsNullOrEmpty(FuelTankBox.Text) && int.TryParse(FuelTankBox.Text, out amountOfFuel) && !FuelTankBox.Text.Contains('-') && selectedFuelTank != null)
{
selectedFuelTank.AddFuelToTank(amountOfFuel);
tankstelle.SaveFuelTanks();
SelectFuelTank(selectedFuelTank);
FuelTankBox.Text = "";
}
}