Not able to get the wpf templated combobox items using test stack white

[SaiPasumarthy]

We are using Test stack white to automate our wpf application.
I'm trying to get the items in templated combobox. The combobox sytle and combobox usage given below.

Template Code:

<Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ComboBox}">
                    <Border x:Name="myEDVTypeComboboxBorder"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}">
                        <Grid x:Name="myEDVTypeComboboxGrid" SnapsToDevicePixels="True" IsSharedSizeScope="True"
                              Background="{DynamicResource  EDVTypeComboBoxStyle.Background}">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*" />
                                <ColumnDefinition Width="Auto" />
                            </Grid.ColumnDefinitions>
                            <Popup x:Name="myEDVTypeComboboxPopup" AllowsTransparency="True"
                                   IsOpen="{Binding Path=IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}"
                                   PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}"
                                   Placement="Bottom" Margin="1">
                                <Border x:Name="myEDVTypeComboboxPopupBorder"
                                        Background="{DynamicResource EDVTypeComboBoxStyle.Background}"
                                        BorderBrush="{DynamicResource EDVTypeComboBoxStyle.BorderBrush}"
                                        BorderThickness="1.1">
                                    <ListBox x:Name="myEDVTypeListBox"
                                             SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                             Width="{TemplateBinding ActualWidth}"
                                             ItemsSource="{TemplateBinding ItemsSource}"
                                             SelectedItem="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedItem}"
                                             Style="{StaticResource DataTypeDescriptionListBoxStyle}" />
                                </Border>
                            </Popup>
                            <ContentPresenter x:Name="myEDVComboboxContentPresenter" Grid.Column="0"
                                              IsHitTestVisible="True"
                                              SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                              HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                              VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                              Margin="{TemplateBinding Padding}"
                                              Content="{TemplateBinding SelectionBoxItem}"
                                              ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                              ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" />
                            <ToggleButton x:Name="myEDVComboboxExpanderArrowToggleButton" Grid.Column="1"
                                          Background="{TemplateBinding Background}"
                                          BorderBrush="{TemplateBinding BorderBrush}"
                                          Style="{StaticResource EDVTypeComboBoxReadonlyToggleButtonStyle}"
                                          IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" />
                            <ToggleButton x:Name="myEDVComboboxExpanderToggleButton"
                                          Background="{TemplateBinding Background}"
                                          BorderBrush="{TemplateBinding BorderBrush}"
                                          Style="{StaticResource EDVTypeComboBoxExpanderToggleButtonStyle}"
                                          IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" />
                        </Grid>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>

Usage :

<ComboBox x:Name="myMM_GEV_TypeComboBox" Grid.Column="1"
                                              ItemsSource="{Binding Source={StaticResource DataTypeFromEnum}}"
                                              SelectedValue="{Binding SelectedBindingGroupElement.DataType, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                                              Style="{StaticResource EDVTypeComboBoxStyle}"
                                              AutomationProperties.AutomationId="MM_GEV_TypeComboBox" />

and i'm trying to get the combobox element items by using below code:

             `var aTypeComboBox = AppHandler.AppWindow.Get<ComboBox>(SearchCriteria.ByAutomationId("MM_GEV_TypeComboBox"));`

aTypeCombBox is exists but inside items are coming zero.

Any help would be appreciated.