Fix possible NPE on delayed Runnable

This is a follow up of I776cabaeaf41.

Bug: 111361570
Test: Pass all presubmit tests in WmTests
  $ atest --test-mapping frameworks/base/services/core/java/com/android/server/wm

Change-Id: Ia8827ae205b6795ddca49ffa152e8de17d0eb8a3
1 file changed