为什么hc的SideMenu组件数据会渲染不出来

[kimValue]

<Grid Width="300" Background="Aqua">
    <Grid>
        <!--<ListBox ItemsSource="{Binding Items}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Header}"></TextBlock>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>-->
        <!--<hc:SideMenu>
            <hc:SideMenuItem Header="首页"></hc:SideMenuItem>
        </hc:SideMenu>-->
        <hc:SideMenu Background="Red" Foreground="Black" ItemsSource="{Binding Path=Items}">
            <hc:SideMenu.ItemTemplate>
                <DataTemplate>
                    <hc:SideMenuItem Foreground="Black" Header="{Binding  Path=Header}"></hc:SideMenuItem>
                </DataTemplate>
            </hc:SideMenu.ItemTemplate>
        </hc:SideMenu>
    </Grid>
</Grid>

public class MainWindowViewModel : BindableBase
{
private ObservableCollection _items;

public ObservableCollection<SideMenuItem> Items
{
    get { return _items; }
    set
    {
        SetProperty(ref _items, value); 
        RaisePropertyChanged();
    }
}

public MainWindowViewModel()
{
    Items = new ObservableCollection<SideMenuItem>();
    Items.Add(new SideMenuItem
    {
        Header = "首页"
    });

    Items.Add(new SideMenuItem
    {
        Header = "用户管理"
    });
    
}

}

public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
hcMenu.Items.Add(new SideMenuItem()
{
Header = "首页"
});
}
}
这样添加又可以

难道其实数据有,但是界面不显示? 但我用普通的ListBox 都能渲染出来,但是用后端item.add() 却有效果 求解 (初学者)