NULL pointer dereference

Hello Friends,

     I'm building a class based SV UVC using URM methodology. As usual, I've master agent & slave agent with independent monitors in it.
   Now, in my master bfm I've used build operation and casted a new unit(say "X") inside it which is needed for my protocol. So, i've binded my monitor to this unit also as shown below:

class master_bfm extends urm_bfm;
   master_monitor mon_ref;
  
  `urm_unit_utils_begin(master_bfm)
       `urm_field_object(mon_ref, REFERENCE + PRINT)
  `urm_unit_utils_end


  function new(input string name, input urm_unit parent);
       super.new(name,parent);
  endfunction: new

  virtual function build();
     super.build();
     $cast(X, create_unit("X_unit", X));
     X.build();

     X.mon = mon_ref; // Here I'm binding the monitor in X_unit to the monitor in master_bfm
  endfunction: build

 ........my code..................

endclass: master_bfm
 


If you observe the code, I've created a "mon" reference in the X_unit with same master_monitor unit.
 And I tried accessing the variables of monitor like "reset_asserted" in X_unit. It gives "NULL pointer dereference" during the "ncsim" run. I tried all ways of connecting it properly through references but could not succeed. In fact, I could access those monitor variables inside the master_bfm but not in X_unit.

Can anyone lemme know the solution of it or where I went wrong?

Thanks in advance,
Jalli