Check null on listener to avoid NPE am: 0c0290df74

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/Wifi/+/25791213

Change-Id: I3b28e1b8e03872d11f4a345da3625bc4febe5788
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>