My sister's family was visiting for dinner, and all the adults were in the kitchen doing food-related tasks while the kids played with art & science supplies in the dining room. Several specimen boxes were included in the child-caused debris field, of a vaguely fantastical nature. The one I remember was basically a small Koosh ball of pulsing darkness--an absence of light rather than black object--which was in what was supposed to be a completely sealed and fused clear plastic box. The kids were warned that this was a naturally-occurring ball of negativity and they should be careful when handling it.
They weren't, obviously.
While my dream-self was helping to wash dishes, there was an exclamation of "oh no!" from the dining room, and when the closest adults ran in to check what had happened, the box was sitting empty on the table and there was an already-much-larger shadowing blotch sort of floating across one wall of the room, towards the doorway for the rest of the house.
And my dream-self had a single moment of realization that panic or worry would just feed the ball of negativity, and so I had to be relentlessly happy until we'd managed to somehow corral the thing back into confinement.
I don't remember the details of this part, just that it involved getting everyone to sing a lot of kids songs, most prominently "Jesus loves me, this I know", until the ball of negativity was small enough to grab in my hand and stuff back into its specimen box.
And then I woke up, with the comforting knowledge that I'd managed to literally stave off a nightmare. It's a pretty good feeling with which to start off your day.