I'm trying to have the player have the last word in a specific dialogue setting.

For example the player says "I'll take care of it" I have the goodbye checkbox checked in the response, but it seems like if you don't put any dialogue in for the npc, that conversation option never appears.

Is there something you have to do to handle this? I'm pretty sure I've seen that happen in game.

You can't have a topic info without dialogue from the NPC. Just write a line of dialogue that the NPC can say in response. Like:

"Do hurry. We haven't got long before they attack." Etc.

Then check goodbye and it will boot the player out of conversation after the NPC says the response.

That's unfortunate. But thank you.