WPF ComboBox with CompositeCollection in Usercontrol does not work: SelectedIndex set to -1

I am using MVVM.

I have a CompositeCollection consisting of

• ComboboxItem with 'Select a vendor' as content
• CollectionContainer which is bounded

When I use the ComboBox XAML code directly in my view the SelectedIndex is set to 0 (as expected).

However, when I put the ComboBox XAML code in a Usercontrol and use the control in my view, the SelectedIndex is set to -1. Any idea how to fix this issue, so that I can use the usercontrol?

All my bindings work.

Note:

• when Combobox XAML code is directly in view: the ComboboxConverter sets the Vendor property to null when 'Select a vendor' is selected by the user.

However, when ComboBox XAML code is in a Usercontrol the code does not get in

if (comboboxItem.Content.ToString() == "Select a vendor")
                {
                    //gets here when code is in view <-> code in control
                    return null;
                }

ComboboxConverter

public class ComboboxConverter : IValueConverter
{
   public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {

        var vendor = value as Vendor;
        if (vendor != null)
        {
            return vendor;
        }
        return null;
}
   public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var comboboxItem = value as ComboBoxItem;
        if (comboboxItem != null)
        {
            if (comboboxItem.Content.ToString() == "Select a vendor")
            {
                //gets here when code is in view <-> code in control
                return null;
            }
            return null;
        }

        var vendor = value as Vendor;
        if (vendor != null)
        {
            return vendor;
        }
        return null;
    }
}

VendorControl

<UserControl x:Class="Tool.Controls.VendorControl"
         xmlns:local="clr-namespace:Tool.Controls"
         xmlns:System="clr-namespace:System;assembly=mscorlib"
         xmlns:objects='clr-namespace:Tool.Objects'
         xmlns:converters='clr-namespace:Tool.Converters'
         mc:Ignorable="d" >
<UserControl.Resources>
<converters:ComboboxConverter x:Key='ComboboxConverter' />
</UserControl.Resources>
<Grid>
<ComboBox Name='cmbVendor'
              SelectedItem='{Binding Vendor, Converter={StaticResource ComboboxConverter}, Mode=TwoWay}'
              Grid.Column='1'
              IsSynchronizedWithCurrentItem='True'>
      <ComboBox.Resources>
    <CollectionViewSource x:Key='VendorsCollection'
                          Source='{Binding Vendors}' />            
        <DataTemplate DataType='{x:Type objects:Vendor}'>
          <StackPanel Orientation='Horizontal'>
            <TextBlock Text='{Binding Name}' />
          </StackPanel>
        </DataTemplate>
      </ComboBox.Resources>
      <ComboBox.ItemsSource>
        <CompositeCollection>
          <ComboBoxItem Content='Select a vendor' />
          <CollectionContainer Collection='{Binding Source={StaticResource VendorsCollection}}' />
        </CompositeCollection>
      </ComboBox.ItemsSource>
    </ComboBox>

ViewModel

private void OnWindowLoaded()
    {
         LoadVendors();
    }

    ObservableCollection<Vendor> _vendors = new ObservableCollection<Vendor>();
    public ObservableCollection<Vendor> Vendors
    {
        get
        {
            return _vendors;
        }

    }


    private Vendor _vendor;
    public Vendor Vendor
    {
        get
        {
            return _vendor;
        }

        set
        {
            if (value != _vendor)
            {
                _vendor = value;
                RaisePropertyChanged(nameof(Vendor));
            }
        }
    }

private void LoadVendors()
    {
            var dVendors = VendorHelper.GetVendors();

            if (Vendors.Count > 0)
            {
                DispatcherHelper.CheckBeginInvokeOnUI(() => Vendors.Clear());
            }

            dVendors.ForEach(dV =>
            {
                var vendor = new Vendor(dV);
                DispatcherHelper.CheckBeginInvokeOnUI(() => Vendors.Add(vendor));
            });
    }