" uvm_do_on_with " constraint issue

In reply to tfitz:
Hi Tom,
I use the method you suggested, in my sequence, I a child sequence, I create/start it as below

        ts_mb_seq = ts_mb_base_sequence_c::type_id::create("ts_mb_seq");
        ts_mb_seq.randomize() with {req.crc24_error == FALSE;};
        ts_mb_seq.start(p_sequencer.ts_mb_sequencer);

I got a run time randomization error with ncsim because of a Null ptr reference.
I understand that at this point, the req of the ts_mb_seq isn’t created yet. How can I get around this, or is this just a ncsim specific issue?
The error below. Thanks.

        ts_mb_seq.randomize() with {req.crc24_error == FALSE;};
                                     |

ncsim: *E,RNDCNSTE (…/uvcs/plc_gen_uvc/src/sequences/plc_base_sequence.sv,110|42): Randomization constraint has this error, which will cause the randomize function to return 0 and no new rand values will be set:
Null handle references in constraints are not supported.
27670 NS + 29 (stop Randomize: Randomize failure)