I know this is so late, but I was wondering about this myself recently. Native speakers can probably tell us better, but from what I have been able to find, the equivalent of the English pleasantry "think nothing of it" could be Det var inget or det var så lite; I've also heard Ingen fara and Ingen orsak (respectively, no danger and no reason, which sound a little more casual to me), and det är lugnt- that seems to be more for when someone apologises to you, rather than thanks you.
So sorry but the bug seems to be more general than that – sometimes accepted answers aren't accepted, it doesn't seem to be tied to contractions specifically. It's the annoying kind of bug that doesn't occur regularly or in specific situations, but seemingly randomly which makes it all the harder to fix I guess.
Oddly enough, this is one of the very few examples where I could imagine myself saying on, as in "Don't think on it" to specifically mean not to muse or reflect on something. Duo doesn't accept that answer, and probably shouldn't, because it's definitely non-standard, but it popped into my head.