ONU Adapter crashes while sending the ONU Deleted Event

The ONU Deleted Event contains the serial number of the OLT, and when the ONU Adapter tries to find the serial number from the core, it does not get back a response. With the current code there is no handling of the null response and so the ONU Adapter crashes.

The core sometimes does not send the response because of a race condition. As part of the OLT Deletion, the core removes the context of the ONU devices from the core itself as well as passes this indication to the ONU Adapter. When the ONU adapter comes back to the core to find out the OLT serial and by that time the ONU is removed from the core, then the core does not send back the response.