It turns out that I missed the null checking in the Case class.

private static void OnPropChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is Case _case && _case.SwitchCase != null && _case.SwitchValue != null)
    {
        _case.Content = _case.SwitchCase.ToString() == _case.SwitchValue.ToString() ? _case.SwitchContent : null;
    }
}