effects/private: Add another guard for multi-screen Drag&Drop

Unlike D&D actions, this method does not return any feedback as to whether an item drop was accepted by any other screen, and so whether it is going through destruction process already, so the best we can do is add another typeof/undefined check after calling that method.

Fixes the following error:

file:///usr/lib/qml/org/kde/kwin/private/effects/WindowHeapDelegate.qml:415:
TypeError: Cannot read property 'deleteDND' of undefined