WPinternals/Views/ContextView.xaml

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<UserControl x:Class="WPinternals.ContextView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:WPinternals"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="700">
    <ContentControl Content="{Binding SubContextViewModel}">
        <ContentControl.Style>
            <Style TargetType="{x:Type ContentControl}">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Content, RelativeSource={x:Static RelativeSource.Self}}" Value="{x:Null}">
                        <Setter Property="Template">
                            <Setter.Value>
                                <ControlTemplate>
                                    <local:Empty/>
                                </ControlTemplate>
                            </Setter.Value>
                        </Setter>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </ContentControl.Style>
    </ContentControl>
</UserControl>