System.Windows.Point clickablePoint = new System.Windows.Point(-1,-1);
object prop = autoElement.GetCurrentPropertyValue(AutomationElement.ClickablePointProperty);
// Do not attempt to cast prop if it is null.
if (prop is System.Windows.Point)
{
    clickablePoint = (System.Windows.Point)prop;
}

System.Windows.Point clickablePoint1;
object clickablePointNoDefault =
    autoElement.GetCurrentPropertyValue(AutomationElement.ClickablePointProperty, true);
if (clickablePointNoDefault == AutomationElement.NotSupported)
{
    // TODO Handle the case where you do not wish to proceed using the default value.
}
else
{
    clickablePoint1 = (System.Windows.Point)clickablePointNoDefault;
}