Fix NPE in Element.recursivelyClearPlacement()

This CL fixes an NPE that I have seen twice while playing with the
VeryBouncy spring configuration. Unfortunately I couldn't find a way to
reproduce (and therefore test it), but this should fix it.

Bug: 290930950
Test: None. See description.
Flag: com.android.systemui.scene_container
Change-Id: Ie3c17c405e07edf03c6b9576f724bfb28fb3de8b