WASHINGTON — China said Thursday it had registered a formal protest over President Joe Biden’s reference to Chinese leader Xi Jinping as a “dictator,” while Treasury Secretary Janet Yellen stressed the critical importance of the two rival powers maintaining relations despite their disagreements. Biden’s remarks at a 2024 reelection fundraiser Tuesday opened a new rift just after Secretary of State Antony Blinken concluded a visit to Beijing that was meant as a step toward stabilizing ties and improving communications with China. Biden called the Chinese president a dictator, depicted him as out-of-touch during last winter’s tumult over a Chinese spy balloon, and dismissed China as having “real economic difficulties.” China’s embassy in Washington said it had delivered a formal protest, with Chinese Ambassador Xie Feng telling senior White House and State Department officials Wednesday that Washington “should take earnest actions to undo the negative impact” of what Biden said or “bear all the consequences.” “With the latest irresponsible remarks about China’s political system and its top leader, people cannot help but question the sincerity of the U.S.