Additional Study Information: The purpose of this study is to determine the safety and effectiveness of the study drug nirogacestat for the treatment of desmoid tumors/aggressive fibromatosis (DT/AF). One group will receive the study medication, the other group will receive a placebo. After the initial phase of the study, participants may elect to participate in another phase of the study in which all participants receive the nirogacestat. Nirogacestat blocks certain signals in the cell that may be responsible for the growth of desmoid tumors (soft tissue sarcoma).