Fix null-dereference when VR navigator is detached

Added in a nullptr check for when the navigator has a detached window.

Bug: 766498
Change-Id: Ia103761f3673b518428f9e34f6b7082d952b3407
Reviewed-on: https://chromium-review.googlesource.com/685818
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Anna Maria <offenwanger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504780}
2 files changed