Add null check for event and event creator in umpire_phone

tournaments/models/tournament.py

@@ -1770,7 +1770,9 @@ class Tournament(BaseModel):
     def umpire_phone(self):
         if self.umpire_custom_phone is not None:
             return self.umpire_custom_phone
+        if self.event and self.event.creator:
             return self.event.creator.phone
+        return None
 
     def calculate_time_to_confirm(self, waiting_list_count):
         """