I would like to set mandatory field backgrounds are red and others are green. So I try to implement below. But I could not set ValueConstraint Nullable property with trigger. Could you help please ?

<Window x:Class="TriggerGirilmesigerekenalanlar.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:y="http://infragistics.com/Editors"
    Title="Window1" Height="300" Width="300">
    <Window.Resources>
        <Style TargetType="{x:Type y:XamTextEditor}">
            <Style.Triggers>
                <Trigger Property="ValueConstraint" Value="{x:Null}">
                    <Trigger.Setters>
                        <Setter Property="Background" Value="green"></Setter>
                    </Trigger.Setters>
                </Trigger>
                <Trigger Property="y:ValueConstraint.Nullable" Value="false">
                    <Trigger.Setters>
                        <Setter Property="Background" Value="red"></Setter>
                    </Trigger.Setters>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <StackPanel>
        <y:XamTextEditor>
            <y:XamTextEditor.ValueConstraint>
                <y:ValueConstraint Nullable="False" ></y:ValueConstraint>
            </y:XamTextEditor.ValueConstraint>
        </y:XamTextEditor>
        <y:XamTextEditor></y:XamTextEditor>
    </StackPanel>
</Window>