show a error message creating placeholder device

ereuse_devicehub/inventory/views.py

@@ -261,10 +261,13 @@ class DeviceCreateView(GenericMixin):
             next_url = url_for('inventory.devicelist')
             if lot_id:
                 next_url = url_for('inventory.lotdevicelist', lot_id=lot_id)
+                if snapshot and snapshot.device:
                     lots = self.context['lots']
                     lot = lots.filter(Lot.id == lot_id).one()
                     lot.devices.add(snapshot.device)
                     db.session.add(lot)
+                else:
+                    messages.error('Sorry, the device could not be created')
 
             db.session.commit()
             messages.success('Device "{}" created successfully!'.format(form.type.data))