Add null check for input receiver before updating batching

- A change in the system (ie. sysui crash or nav mode change) could
  cause the input monitor to be disposed before the swipe animation
  settles

Bug: 170121063
Test: Kill sysui while swiping up

Change-Id: I1417b109fecdb98fae6197c7038dbe9307470853
1 file changed