Since lifetime of a KWindowShadow is a child of QWindow, we nenot strictly bounded to use a guardedlifetime
pointer to store the shadow in orderof the associated widget, we need to prevent accessing or deletingunregister the shadow when it's
dangling pointers in ShadowHelper::_shadows. It's quite possible thatdestroyed in order to prevent accessing or deleting dangling pointers
one could destroy the associated widget and with it the QWindowafterwards.