fix

rowers/models.py

@@ -3778,9 +3778,12 @@ def update_duplicates_on_delete(sender, instance, **kwargs):
                 t = ww.duration
                 delta = datetime.timedelta(
                     hours=t.hour, minutes=t.minute, seconds=t.second)
-                enddatetime = ww.startdatetime+delta
-                if enddatetime > d.startdatetime:
-                    ws2.append(ww)
+                try:
+                    enddatetime = ww.startdatetime+delta
+                    if enddatetime > d.startdatetime:
+                        ws2.append(ww)
+                except TypeError:
+                    pass
 
             if len(ws2) == 0:
                 d.duplicate = False