WPF ItemsPanelTemplate不允许添加简单的UIElement

Question

我试图设置Items控件的样式并使用此XAML：

<DataTemplate x:Key="DatatemplatePlateItemContainer">

    <Grid x:Name="PlateItemContainer">

        <!-- Does recursive ItemTemplate selection        -->
        <ItemsControl ItemsSource="{Binding Path=Children}"
                        ItemTemplateSelector="{StaticResource TemplateselectorPlateItem}">

            <ItemsControl.ItemContainerStyle>
                <Style>
                    <Style.Setters>
                        <Setter Property="Grid.Row" Value="{Binding Path=GridRow}"/>
                        <Setter Property="Grid.Column" Value="{Binding Path=GridColumn}"/>
                    </Style.Setters>
                </Style>
            </ItemsControl.ItemContainerStyle>

            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <Grid ShowGridLines="True">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="Auto"/>
                        </Grid.RowDefinitions>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>

                        <!-- This line will not let the ItemsControl to load -->
                        <Rectangle Grid.Row="0" Grid.Column="0" Grid.RowSpan="1" Grid.ColumnSpan="4"/>

                    </Grid>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
    </Grid>
</DataTemplate>

如果我尝试在其ItemsPanelTemplate.Grid中添加任何UIElement（此示例中的Rectangle） - 从底部添加第7行 - 它将失败。只是评论一下 - 它的确有效。这个简单的XAML有什么问题？它不会提供太多信息 - 仅在输出中，我无法追踪：

A first chance exception of type 'System.InvalidOperationException' occurred in PresentationFramework.dll
A first chance exception of type 'System.Xaml.XamlObjectWriterException' occurred in System.Xaml.dll
A first chance exception of type 'System.Windows.Markup.XamlParseException' occurred in PresentationFramework.dll
Additional information: Add value to collection of type 'System.Windows.Controls.UIElementCollection' threw an exception.
An unhandled exception of type 'System.Windows.Markup.XamlParseException' occurred in PresentationFramework.dll
Additional information: Add value to collection of type 'System.Windows.Controls.UIElementCollection' threw an exception.