Add null pointer protection of getContext() in onEngineShown(). am: 86fd55b92a am: 77b7a806a7

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/WallpaperPicker2/+/12041870

Change-Id: I776b78b6a8f1055208d1cc6d4f98c1dc8299ed8d