How to fix: COM/object handle is null

Hello Folks,
I am using Amibroker through IB controller + Interactive Broker TWS (API) + Data feed from RTDF
But my AFL is happened the below code:
ibc = GetTradingInterface("IB");
if( ibc.IsConnected() )
------------------^
COM/object handle is null
Use 'Edit Formula' to correct error

If I turn off the RTDF or data feed from TWS, my AFL is fine.
Any idea and could you kindly suggest how to fix it...Thanks!