Why is the ending of Huck Finn controversial?

Could you elaborate on why the ending of Mark Twain's classic novel, The Adventures of Huckleberry Finn, has sparked such controversy? What specific elements of the conclusion have drawn criticism and debate among readers, scholars, and critics? How does the ending reflect on the themes and messages of the novel as a whole, and what implications does it have for contemporary discussions on race, identity, and morality?