[SideSheet] Prevent NullPointerException when viewRef was nullified and only then accessed by runAfterLayout lambda

[eugenegff]

Prevent NullPointerException when viewRef was nullified as part of SideSheetBehavior.onDetachedFromLayoutParams() and only then accessed by runAfterLayout lambda. Similar nearby checks outside of lambda are not enough, as all of them are executed immediately, and access in lambda is done later, after layout.

Issue is rare enough, but occurs periodically in the wild

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[eugenegff]

signed cla