I have bad Betta dreams too /= I can't remember most of them, but I dreamed once that Neon struggled around with all of his fins melted off )=> I know I have this fear of him getting health problems, so maybe that's why. But let me tell you, a Betta with no fins, especially one that you love, is not something you want to see. Luckily that is pretty much impossible unless a Betta has severe fin rot and is with another male Betta, and he tailbites, and he has super prickly decor.
Think of it this way, if the dream is impossible, the only real meaning of it was you fear it will happen. Like I doubt your Bettas tank will suddenly start draining magically as tanks are built strong. Don't worry about it, it'll never happen. Just don't think about it, and it will eventually fade from your head.