fix(adorner): guard against null instance in AdornerElement.OnInstanceChanged (closes #239)

[omid-io]

Summary

Fixes #239 where switching or unbinding a DataTemplate containing a control with an AdornerElement (such as PopTip.Instance) throws a NullReferenceException in AdornerElement.OnInstanceChanged.

Root Cause

In AdornerElement.cs:

private static void OnInstanceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is not FrameworkElement target) return;
    var element = (AdornerElement) e.NewValue;
    element.OnInstanceChanged(target);
}

When e.NewValue is null (e.g. when a DataTemplate is unloaded, cleared, or rebound), (AdornerElement) e.NewValue evaluates to null, causing element.OnInstanceChanged(target) to throw a NullReferenceException.

Solution

Safely pattern-match and guard e.NewValue:

private static void OnInstanceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is not FrameworkElement target) return;
    if (e.NewValue is AdornerElement element)
    {
        element.OnInstanceChanged(target);
    }
}

Verification

• Clean build of HandyControl_Net_GE45.csproj targeting net8.0-windows with 0 warnings and 0 errors.