the NK was created with dragon glassAnd no one knows for sure if dragon glass or Valyrian steel even harms the night king it's all speculation.
He created the white walkers doesn't mean he has their same weaknesses
he needs some sort of weakness. can't have him be immune to conventional weapons, dragon glass, valyrian steel, and dragon fire, but weak to... butterscotch?
maybe him and Bran will end up having a 'battle of the minds'