By Michael Rosenberg The NBA has seen how ugly player-fan altercations can get. Together, the league and its players need to make sure they don’t get that ugly again. For decades, the NBA has thrived by packing fans and superstars into tight spaces, creating the best game atmosphere of any pro sports league. It’s good business to have fans within a stone’s throw of superstars—you know, as long as they don’t actually throw stones. No one is throwing stones just yet. But over the past couple of weeks, four future Hall of Famers have publicly complained about fan behavior. Kyrie Irving (in Boston…