How do I show grouped item headers on a datagridtemplatecolumn with a grouped combobox

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VB

WPF

DataGrid

I have a DataGridTemplateColumn with a Grouped combobox. The Combobox has as ItemSource a ListCollectionView with two fields: TipoBienes_ID and Bienes_ID.

What I want is simple, a combo within a datagrid column that allow me to pick one Bienes_ID but groups by TipoBienes_ID like this:

TipoBienes_ID1
Bienes_ID1
Bienes_ID2
TipoBienes_ID2
Bienes_ID3
Bienes_ID4

I have managed to view the items correctly grouped, select the one I want, but instead of the headers a light blue space appears (as it should, but without the header text). I know I have a Binding problem but cannot find it. I am sure it is a simple thing that I am missing. I am binding the datagrid in code (see below) and the combobox within the datagridtemplatecolumn with a property in the main window.

Here is my HTML code:

<DataGrid Grid.Row="1" Name="DetalleFactura" AutoGenerateColumns="False" >
            <DataGrid.Columns >
             <DataGridTemplateColumn x:Name="ColumnaCombo">
                    <DataGridTemplateColumn.CellTemplate >
                        <DataTemplate >
                            <TextBlock Text="{Binding Path=Bienes_ID}"></TextBlock>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                    <DataGridTemplateColumn.CellEditingTemplate >
                        <DataTemplate >
                            <ComboBox x:Name="ComboBienesAgrupados"  SelectedValuePath="Bienes_ID" ItemsSource="{Binding Path=ListaBienes, RelativeSource={RelativeSource AncestorType=Window}}" SelectedValue="{Binding Bienes_ID}"  >
                                <ComboBox.GroupStyle >
                                    <GroupStyle>
                                        <GroupStyle.HeaderTemplate >
                                            <DataTemplate >
                                                <TextBox  Text="{Binding TipoBienes_ID}" Background ="AliceBlue" ></TextBox>
                                            </DataTemplate>
                                        </GroupStyle.HeaderTemplate>
                                    </GroupStyle>
                                </ComboBox.GroupStyle>
                                <ComboBox.ItemTemplate >
                                    <DataTemplate >
                                        <TextBlock Text="{Binding Path=Bienes_ID}"   ></TextBlock>
                                    </DataTemplate>
                                </ComboBox.ItemTemplate>
                            </ComboBox>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellEditingTemplate>
                </DataGridTemplateColumn>
                               </DataGrid.Columns>
        </DataGrid>

Here is the part that populates the datagrid and the combo:

 Public Class Bien
    Public Property Bienes_ID As String
    Public Property TipoBienes_ID As String
End Class

Public Class Bienes

    Inherits CatalogoWPF
    Sub New()
        MyBase.New(My.Settings.CadenaArchivo, "Select * from Bienes order by Bienes_ID", "Bienes")
    End Sub


    Public Property Bienes_ID As String
        Get
            Return ObtainField("Bienes_ID")
        End Get
        Set(value As String)
            SetField("Bienes_ID", value)
        End Set
    End Property

    Public Property TipoBienes_ID As String
        Get
            Return ObtainField("TipoBienes_ID")
        End Get
        Set(value As String)
            SetField("TipoBienes_ID", value)
        End Set
    End Property

    Function ObtieneListaAgrupada() As ListCollectionView
        Dim Lista As New List(Of Bien)
        Dim i As Integer
        Dim elemento As Bien
        Dim salida As ListCollectionView
        Navigate(PossibleDirections.FirstItem)
        For i = 0 To Count - 1
            elemento = New Bien
            elemento.Bienes_ID = Bienes_ID
            elemento.TipoBienes_ID = TipoBienes_ID
            Lista.Add(elemento)
            Navigate(PossibleDirections.NextItem)
        Next
        salida = New ListCollectionView(Lista)
        salida.GroupDescriptions.Add(New PropertyGroupDescription("TipoBienes_ID"))
        Return salida
    End Function
End Class

In the window where the datagrid resides, I have this code in the Loaded Event:

Public Class frmCapturaFacturas
    Implements INotifyPropertyChanged
    Dim m_Proveedores As Proveedores
    Dim m_DetalleFacturas As DetalleFacturas
    Dim m_DetalleImpuestosFacturas As DetalleImpuestosFacturas
    Dim m_impuestosFacturas As DetalleImpuestosFacturas
    Dim m_impuestos As ImpuestosFacturas
    Dim m_marcas As Marcas
    Dim m_bienes As Bienes
    Dim m_tablaDetalle As DataTable
    Dim m_tablabienes As DataView
    Dim m_SubTotalFactura As Decimal
    Dim m_SubTotalImpuestos As Decimal

    Dim m_ListaBienes As ListCollectionView
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged

   
    Public Property ListaBienes As ListCollectionView
        Get
            Return m_ListaBienes
        End Get
        Set(value As ListCollectionView)
            m_ListaBienes = value
        End Set
    End Property

    Private Sub frmCapturaFacturas_Loaded(sender As Object, e As RoutedEventArgs) Handles Me.Loaded


        m_bienes = New Bienes()
               ListaBienes = m_bienes.ObtieneListaAgrupada
        m_bienes.Dispose()

        m_DetalleFacturas = New DetalleFacturas
        m_tablaDetalle = m_DetalleFacturas.CreaTabla

        DetalleFactura.ItemsSource = m_tablaDetalle.DefaultView
    End Sub

What I have tried:

I have replaced Text="{Binding Path=TipoBienes_ID}" with a plain Text="hello" and on every header that word appeard. Also I replaced Text="{Binding Path=TipoBienes_ID}" with Text="{Binding Path=Bienes_ID}" and the same blank space was shown.