Fix RTT NPE due to ImsCall#close being called am: fff67ce3fc am: d8d83258d6

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/opt/net/ims/+/18973189

Change-Id: Ifb0626828cdcdc4287ebdfeb8efd9d78cc8932ea
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>