One of the most radical and under the radar groups with close ties to a nonprofit run by tech millionaire Neville Roy Singham is a pro-North Korea nonprofit that bashes Uncle Sam and plots the demise of US relations with South Korea from its home base in Midtown, The Post has learned. Nodutdol, which means stepping stone in Korean, claims it was formed to promote “US-Korean understanding and education,” according to its nonprofit tax filings. But in reality, the group has worked to radicalize