There is no "nothing-at-stake" problem. No entity can ever buy 51% of the coins in existance.
The attacker needs 51% of coins the existed
at some point in the past. He can attack the network with 0% of the coins that are currently considered as valid. That's why it's called "nothing at stake": he can attack the network using coins that were already spent!
Are you sure there was a case like this in NXT history? Because if there was not one, as far as I understand N@S is not applicable @NXT..