Steps to reproduce:

1. open /demos/form-control/lookup.php
2. in Lookup countries section (1st section after the first lookup field) click Save
3. when submit atk4/data generates Must not be null validation exception, but this exception is currently caught by

   ui/src/Form.php

   Line 263 in 393dfc5

   } catch (\Atk4\Data\ValidationException $e) {

   and the generated JS that should set the error message below the field is broken

Currently no validation exception message is shown and Save does "nothing visible to the user".

Expected result:

In console I see:

maybe related

I am marking this issue as MAJOR as this issue might affect/hide all validation exceptions in (only nested?) forms and app user has no feedback why Save does not work.

The fix must cover this with Behat test.