What is the main problem in Huckleberry Finn?

Could you elaborate on the key issue presented in Mark Twain's Huckleberry Finn? Are we discussing the portrayal of slavery and its moral implications, or perhaps the complex relationship between Huck and Jim, and the challenges they face as they traverse the Mississippi River? Or is there another, more nuanced aspect of the novel that serves as its primary problem? I'm intrigued to hear your thoughts on this matter.