Robert Stigwood, who managed the Bee Gees at the peak of their career and produced one of the defining films of the 1970s in “Saturday Night Fever”, has died at the age of 81.Stigwood’s death was announced on Facebook Monday by Spencer Gibb, a son of the late Bee Gees singer Robin Gibb. Spencer Gibb described Stigwood as a “creative genius with a very quick and dry wit”.