Fix an NPE in onTopologyChanged

I'm seeing an NPE here when the app handle is showing for bubbles
on foldable. Marking topology as potentially null and return
when it is.

Flag: EXEMPT null check
Test: manual - turn on enable_bubble_anything flag and open an app
Bug: 398867440
Change-Id: I5819295315f19a16886f2a834f94c0162094c314