[@Sophrus] Wraith has it. Urgrugg's staff can store a very small amount of warp energy. Null fields don't actually dissipate warp energy that is inside of them, they just disconnect the warp from the area of the materium they cover. As long as you have something that can hold warp energy, such as an enchanted item or a Thousand Son's rank and file marine, then you're still able to use it.

It's important not to confuse a 'null zone' with the field created by a 'null,' also known as 'psychic blanks.' Nulls actively dissipate warp energy, which is why things like culexus assassins are so dangerous.

If the null field the characters are in was the same as the kind created by a blank, especially one strong enough to block the energy of tanks full of eldar spirit stones, they would all very quickly go completely insane. And not the fun kind, either. The 'curl up on the floor and chant gibberish while you gnaw your lip' kind of insane.