Remove DataContext
binding from your ContextMenu
:
<ContextMenu x:Key="SimpleDataObjectContextMenu" >
<MenuItem Header="Create" Command="{Binding AddRelation}" CommandParameter="{Binding Parent.PlacementTarget.Name, RelativeSource={RelativeSource Self}}"/>
<MenuItem Header="Delete" Command="{Binding DeleteRelation}" CommandParameter="{Binding Parent.PlacementTarget.Name, RelativeSource={RelativeSource Self}}"/>
</ContextMenu>