If you are trying to constrain an effector to a null, here is a little trick I use, so the effector doesn't have to move. Effect/Nullifier works, but it creates another Null at 0,0,0 and makes it the parent.
Anyway, here is a simple solution.
1. Create you null anywhere. Go ahead and make it the correct size.
2. Select the Null and use a positional constraint to place it on the effector.
3. Now use Constraint/Relax to remove the constraint.
4. Select the effector and use the positional constraint to constrain it to the null.
That's it.
This technique is a quick way to get objects to line up etc. Just remember to relax the constraint afterwards.
Hope this is helpful,
Urp