In reply to shankar_logic:
Considering you have done class and module instantiations correctly.
Issue is with uvm_config_db:: get method,
As inside aes_agent_bfm uvm_config_db::set() method shares resources with name
1)aes_agent_bfm.driver
2)aes_agent_bfm.monitor
While uvm_config_db::get() method tries to access them with name-
1)top.aes_agent.driver
2)top.aes_agent.monitor
These field_name should be same as that of used with uvm_config_db::set().