GitLab

ShelfController outlives TabletModeController. However, it looks like
Shell destruction can trigger display layout changes, which are
observed by ShelfController. These may be dependent on screen rotation
animation state, and hence are hard to repro.

Add a null check to avoid the crash, since this is simple and safe to
backport. A followup CL will clean up the shutdown ordering.

Bug: 817522
Change-Id: I7d02faf6179768457e56dff8bfdf2a74c8701b4c
Reviewed-on: https://chromium-review.googlesource.com/966785Reviewed-by: Weidong Guo <weidongg@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543748}