Fix runtime crash of using null wallpaperConnection instance am: 2c1d7f41e1 am: d831789d79

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/ThemePicker/+/11683412

Change-Id: Icae5b374e62f43e3fb68a58c33f224f97283645c