pass data from the datagrid of one form to the textbox of another form wpf

I have the following code to load my data into a datagrid of a form

Private Sub cargarGrid()
    Dim conexion As New ConexionBD
    Dim data As DataTable
    dataGridVehiculos.ItemsSource = Nothing
    Try
        data = conexion.mostrarDatosenGrid("VEHICULOS_EN_TRANSITO")
        dataGridVehiculos.ItemsSource = data.DefaultView
    Catch ex As Exception
        MsgBox(ex.ToString)
    End Try
End Sub

Public Function mostrarDatosenGrid(ByVal tabla As String) As DataTable
        Dim nuevaTabla As New DataTable
        Try
            conexion()
            _adaptador.SelectCommand = New SqlCommand("SELECT * FROM " & tabla, _conexion)
            _conexion.Open()
            _adaptador.SelectCommand.Connection = _conexion
            _adaptador.Fill(nuevaTabla)
            Return nuevaTabla
        Catch ex As Exception
            MsgBox(ex.Message)
            Return Nothing
        Finally
            _conexion.Close()
        End Try
    End Function

It works for me to load the data in the data grid, what did not work for me is that when selecting an item from the data grid it does not load me in the texbox of another form I have the following code.

Private Sub dataGridVehiculos_SelectionChanged(sender As Object, e As SelectionChangedEventArgs) Handles dataGridVehiculos.SelectionChanged
    Dim tabla As DataRowView = DirectCast(dataGridVehiculos.SelectedItem, DataRowView)
    registro.noTiquete = tabla.Item(1)
    registro.fechaPesoVacio = tabla.Item(2)
    registro.fechaPesoLleno = tabla.Item(3)
    registro.placa = tabla(4)
    registro.conductor.cedula = tabla.Item(5)
    registro.conductor.nombre = tabla.Item(6)
    registro.clienteProveedor.nombre = tabla.Item(7)
    registro.productoMateriaPrima.nombre = tabla.Item(8)
    registro.tara = tabla.Item(9)
    registro.bruto = tabla.Item(10)
    registro.neto = tabla.Item(11)
    registro.noOrden = tabla.Item(12)
    registro.origen.nombre = tabla.Item(13)
    registro.destino.nombre = tabla.Item(14)
    registro.observaciones = tabla.Item(15)
    registro.taraAutorizado = tabla.Item(16)
    registro.brutoAturorizado = tabla.Item(17)
    registro.usuarioTransito = tabla.Item(18)
    registro.usuarioProcesado = tabla.Item(19)
    registro.rutaFotos = tabla.Item(20)
    registro.horaPesoVacio = tabla.Item(21)
    registro.horaPesoVacio = tabla.Item(22)
    registro.noShipment = tabla.Item(23)
    registro.noSello = tabla.Item(24)
    registro.noR = tabla.Item(25)
    registro.noContenedor = tabla.Item(26)
    registro.planta.nombre = tabla.Item(28)
    registro.transportadora.nombre = tabla.Item(29)
    registro.pesoOrden = tabla.Item(30)
End Sub

I have not been able to know what I'm doing wrong, or if I have to do it in another way, the registration class is a class that contains the necessary fields to store the grid data and from there I can take that information to the texbox, the point is that the data is not moving from the grid to the fields of the class, could someone give me a hand to know exactly how to do it? this the xaml code.

<DataGrid Margin="-351.935,-15.119,-355.73,-61.323" ItemsSource="{Binding Items3}" CanUserSortColumns="True" CanUserAddRows="False" AutoGenerateColumns="False"  md:DataGridAssist.CellPadding="13 8 8 8" md:DataGridAssist.ColumnHeaderPadding="8"></DataGrid>

Thanks for the help.