How to reproduce:

1. open demo /ui/demos/form/form2.php
2. enter blabla in country field (first)
3. click "Check Duplicate"

Result:

Critical Error TypeError:
Atk4\Ui\Demos\{closure}(): Argument #2 ($name) must be of type string, Atk4\Ui\Js\Jquery given,
called in src/View.php on line 924 

Expected result: "This country name can be added."
Source: https://github.com/atk4/ui/blob/develop/demos/form/form2.php

No idea how to fix it, since button click interface is undocumented and differs from onSubmit interface.