如果一个MenuItem有一个自定义的MenuItems并且是在一个单独的XAML文件中定义的，那么如何使用子ControlTemplate？

<ControlTemplate x:Key="CustomMenuItemTemplate" TargetType="MenuItem">
    <Grid>
        <!-- 自定义的MenuItem外观 -->
        <Border Background="LightGray" BorderBrush="Black" BorderThickness="1">
            <TextBlock Text="{TemplateBinding Header}" Margin="5"/>
        </Border>
        
        <!-- 子菜单 -->
        <Popup x:Name="SubMenuPopup" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Right">
            <Grid>
                <!-- 子菜单的内容 -->
                <StackPanel>
                    <TextBlock Text="Submenu Item 1" Margin="5"/>
                    <TextBlock Text="Submenu Item 2" Margin="5"/>
                </StackPanel>
            </Grid>
        </Popup>
    </Grid>
    
    <!-- 鼠标悬停时的效果 -->
    <ControlTemplate.Triggers>
        <Trigger Property="IsMouseOver" Value="True">
            <Setter TargetName="SubMenuPopup" Property="IsOpen" Value="True"/>
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

<Menu>
    <MenuItem Header="Main Item" Template="{StaticResource CustomMenuItemTemplate}"/>
</Menu>