Fixed tapir accounts without linked share_owner not being able to see…

… shift details.

tapir/shifts/models.py

@@ -710,6 +710,7 @@ def user_can_attend(self, user):
             set(self.required_capabilities).issubset(user.shift_user_data.capabilities)
             and self.shift.is_in_the_future()
             and not self.shift.cancelled
+            and hasattr(user, "share_owner")
             and user.share_owner.is_active(self.shift.start_time)
         )