Add a null protection for getContext() when updating preview in theme/grid fragment. am: c4d30d641a am: dd153e530f

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/ThemePicker/+/11683410

Change-Id: I7d80432b68d9ca50fe198ad8974ee0376a5caece