Add null check to umpire_mail method

tournaments/models/tournament.py

@@ -1765,7 +1765,9 @@ class Tournament(BaseModel):
     def umpire_mail(self):
         if self.umpire_custom_mail is not None:
             return self.umpire_custom_mail
-        return self.event.creator.email
+        if self.event and self.event.creator:
+            return self.event.creator.email
+        return None
 
     def umpire_phone(self):
         if self.umpire_custom_phone is not None: