In reply to xfinity:
Your set makes only child components of axi_slave_env with hierarchy as { axi_slave_env.get_full_name(),“*.slave_uvc” } having access to the slave_uvc_cfg.
As per your component hierarchy , user_env is parent component of axi_slave_env .
So essentially you are setting from a child component ( whose build_phase() is called later ) and getting in a parent component ( whose build_phase() is called earlier )
You are doing the get before the set is done ( also the hierarchy for the config via set is incorrect ) which is opposite to what is typically done ( i.e set from Top component and get in lower component )