First, if you’re going to work on installing a light fixture above the kitchen sink, I suggest that you first cover the drain with a towel, newspaper, etc.
If you disregard that advice, in the alternative I suggest that you run the garbage disposal first to clean out nasty gunk that might be lingering there.
After I dropped a screw into the garbage disposal, I couldn’t initially locate it among all the muck. I came up with the “clever” solution of fishing around for it with some strong rare-earth magnets (out of old hard drives) stuck on the end of a screwdriver. It didn’t occur to me that the magnets might be better attracted to the mass of steel comprising the garbage disposal. So now I had a disposal full of muck, a screw, and a loose bunch of rare earth magnets.
I scooped the nasty stuff out with my fingers and found the screw but the magnets remained elusive. The final lesson learned was that the screwdriver shaft was a pretty good probe for finding the magnets – there was a spot in the disposal where the tip of the screwdriver was attracted, and the magnets were there.