RadComboBox drop down list does not close when used in the RadPopup

<Button
            Clicked="ShowPopup"
            HorizontalOptions="Center"
            Text="Show RadPopup"
            VerticalOptions="Start"
            x:Name="button">
            <telerik:RadPopup.Popup>
                <telerik:RadPopup
                    IsModal="True"
                    OutsideBackgroundColor="#6F000000"
                    x:Name="popup">
                    <telerik:RadBorder BackgroundColor="Wheat" CornerRadius="8">
                        <Grid
                            HeightRequest="200"
                            Padding="10"
                            WidthRequest="200">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="30" />
                                <RowDefinition Height="auto" />
                            </Grid.RowDefinitions>
                            <telerik:RadButton
                                AutomationId="CloseButton"
                                Clicked="ClosePopup"
                                Grid.Row="0"
                                HorizontalOptions="End"
                                Padding="2"
                                Text="Close" />
                            <telerik:RadComboBox AutomationId="staticItemsComboBox" Grid.Row="1">
                                <telerik:RadComboBox.ItemsSource>
                                    <x:Array Type="{x:Type x:String}">
                                        <x:String>USA</x:String>
                                        <x:String>Uganda</x:String>
                                        <x:String>Ukraine</x:String>
                                        <x:String>Canada</x:String>
                                        <x:String>France</x:String>
                                        <x:String>Italy</x:String>
                                        <x:String>United Kingdom</x:String>
                                        <x:String>China</x:String>
                                        <x:String>Japan</x:String>
                                    </x:Array>
                                </telerik:RadComboBox.ItemsSource>
                            </telerik:RadComboBox>
                        </Grid>
                    </telerik:RadBorder>
                </telerik:RadPopup>
            </telerik:RadPopup.Popup>
        </Button>

        private void ClosePopup(object sender, EventArgs e)
        {
            popup.IsOpen = false;
        }
        private void ShowPopup(object sender, EventArgs e)
        {
            popup.IsOpen = true;
        }