public class PropertyDummyViewModel : NotifyPropertyChanged
    {
        [DisplayName(@"Double Value Default Editor")]
        [DefaultValue(70)]
        public double DoubleValueDefaultEditor { get; set; }
    }

<Grid ...
    x:Name="PropertyGrid">
    <xctk:PropertyGrid SelectedObject="{Binding Path=DataContext, ElementName=PropertyGrid}" />
</Grid>

public void ShowDialog()
    {
        var model = new PropertyDummyViewModel();
        var view = new PropertyDummyView() { DataContext = model };
        var window = new Window() { Content = view };
        window.ShowDialog();
    }