How to close pop up while pressing alt+tab

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VB

WPF

I have used a pop up for auto complete text box.It is working fine but not closing while pressing alt+tab or opening new window.My xaml code is below-

<StackPanel Margin="0">
       <Label Content="" Width="auto" x:Name="AutocompleteLabel" Style="{DynamicResource LabelStyle}"/>
       <TextBox x:Name="AutoCompleteTextBox" Width="auto" Margin="0,0,5,0" TextWrapping="NoWrap" TextChanged="AutoCompleteTextBox_TextChanged" GotFocus="AutoCompleteTextBox_GotFocus" Style="{DynamicResource TextBoxStyle}" LostFocus="AutoCompleteTextBox_LostFocus"/>
       <Popup IsOpen="True" Placement="Bottom" Name="AutoCompletePopUp" AllowsTransparency="True">
       <ListBox x:Name="SuggestionListBox" Background="#ffffffff" Visibility="Collapsed"  VerticalAlignment="Top" Width="{Binding ElementName=AutoCompleteTextBox}" SelectionChanged="SuggestionListBox_SelectionChanged" HorizontalContentAlignment="Left" Focusable="False" Margin="0,0,5,0"/>
       </Popup>
   </StackPanel>

Plz help me