Revert "Revert "[AC] Fix NPE. If the client sends multiple unregister requests...

Revert "Revert "[AC] Fix NPE. If the client sends multiple unregister requests at the same time, the client request could be null before the next request is processed.""

This reverts commit f470e292.

Reason for revert: Fixed warning from presubmit

Change-Id: I3e1ea17a2e149b548885666d9e8418647b96439e