In reply to jms8:
You are getting a null access error because req is not constructed at the time you randomize from the top level sequence. A_seq_stop_signal does not construct req.
You should be using the UVM factory to add the constraint. The factory was specifically designed to allow you modify lower level classes from the top. I don’t have time right now to give you an example. but what you should do is extend a_tb_txn with the wait_time constraint into another class. then set the override in the body of A_lseq_signal, and then call super.body(). Hopefully this will give you an idea of what to do.