Going by google, the objection seems to be that all the major characters are white, and the most prominent black characters, recurring and one-off, are painted as villains. How accurate that is I have no idea, I tend not to like police procedurals, just giving the side of the argument CFC neglected.
I don't think anyone's saying you shouldn't watch it, or that the show's actively recruiting for the KKK, or anything like that. This whole area of critique is never 'This is irredeemable trash if you like it you are bad and should feel bad,' outside of overexcited kids or the things Crouton mentioned. Just tends to be pointing out awkward trends, so that at the end of the day you can enjoy fiction as fiction rather than accidentally taking on board stereotypes. The reaction shouldn't be "Oh no this is awful BURN IT!" outside of a few extreme cases, just a "Huh... yeah, that does happen a lot. Anyway, next episode!"