Fix crash

tournaments/models/match.py

@@ -185,7 +185,9 @@ class Match(models.Model):
         date = self.formatted_start_date()
         duration = self.time_indication()
         court = self.court_name(self.court_index)
-        group_stage_name = self.group_stage.display_name()
+        group_stage_name = None
+        if self.group_stage:
+            group_stage_name = self.group_stage.display_name()
 
         ended = self.end_date is not None
         livematch = LiveMatch(title, date, duration, court, self.started(), ended, group_stage_name)