We #770 that clears some variables that were causing a memory leak in the bidirectional streaming classes. This change lead to a potential null pointer exception during the shut-down process when the BackgroundConsumer tries to access the removed callback.

The issue would only come up when the background thread failed to finalize within 1 second, and the error would be swallowed internally. But it would result in an error log that should be avoided

This PR does a None check to avoid emitting the error log

Fixes #788