example pfb_sync_test

Hi when i try to run the example i get this error 

ControlPort Monitor running.

python3: /build/gnuradio-6i8Hb6/gnuradio-3.8.0.0~gnuradio~bionic/gnuradio-runtime/lib/controlport/rpcmanager.cc:38: static rpcserver_booter_base* rpcmanager::get(): Assertion `booter_registered || aggregator_registered' failed.