SGraphPin::InvalidateGraphData

Called when ed graph data is cleared, indicating this widget can no longer safely access GraphPinObj Typically an [SGraphPin](API\Editor\GraphEditor\SGraphPin) is destroyed before the ed graph data is actually deleted but because [SWidget](API\Runtime\SlateCore\Widgets\SWidget) is reference counted we have had problems in the past with the [SGraphPin](API\Editor\GraphEditor\SGraphPin) (or Node) living unexpectedly long.