Famous theologians or saints who held that there were no valid Popes during the Great Western Schism? St. Catherine of Siena and St. Vincent Ferrer, for example, disagreed about who the valid Pope was during the Great Western Schism.

Are there famous theologians or even saints who lived during the Great Western Schism and who considered none of the papal claimants valid popes?
According to William Morgan in "The Great Schism of the West and the Catholic Church Today," St. Vincent Ferrer did not recognize any of the claimants towards the end of the schism (source):


  Disaster struck for Papa de Luna when his most important supporter, St Vincent Ferrer, decided that he was pertinaciously dividing the Church, and so was a schismatic and no longer Pope. The miracle working Dominican saint never recognised the Urbanist claimants. Rather was he now a sedevacantist, looking to the General Council to resolve the Schism.