I have created a custom page header for my project in which I have set up a DependencyProperty for CloseButtonCommandProperty. I have added a callback to this Property to set another DependencyProperty (ShowCloseButtonProperty).
The issue I am having is that the null is not being hit, so ShowCloseButton is always set to true.

I am a bit green on dependency properties, so any help (or corrections on my code) are very welcome.

public static readonly DependencyProperty CloseButtonCommandProperty =
    DependencyProperty.Register(nameof(CloseButtonCommand), typeof(AsyncRelayCommand), typeof(WindowHeader),
        new PropertyMetadata(null!, new PropertyChangedCallback(OnCloseButtonCommandChanged)));

public AsyncRelayCommand CloseButtonCommand
{
    get => (AsyncRelayCommand)GetValue(CloseButtonCommandProperty);
    set => SetValue(CloseButtonCommandProperty, value);
}

private static void OnCloseButtonCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is WindowHeader ctrl)
        ctrl.ShowCloseButton = e.NewValue != null;
}

 public static readonly DependencyProperty ShowCloseButtonProperty =
     DependencyProperty.Register(nameof(ShowCloseButton), typeof(bool), typeof(WindowHeader),
         new PropertyMetadata(false));

 public bool ShowCloseButton
 {
     get => (bool)GetValue(ShowCloseButtonProperty);
     private set => SetValue(ShowCloseButtonProperty, value);
 }