Called when ed graph data is cleared, indicating this widget can no longer safely access GraphPinObj Typically anSGraphPinis destroyed before the ed graph data is actually deleted but becauseSWidgetis reference counted we have had problems in the past with theSGraphPin(or Node) living unexpectedly long. If such a situation occurs the code attempting to own theSWidgetshould be demoted to a WeakPtr, but for added safety we track graph data invalidation here: