Fix crash when entering overview from home launcher.

The overview's shield widget can be created async, thus
it can be null.

Bug: 942622
Test: manual
Change-Id: Ib39ccd4dff02b67ad08bb280e8a3dd803a478673
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525166
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641369}
1 file changed