UI for adding field is broken.

ab.shakir → made their first commit to this issue’s fork.

I thought it is worth debugging this issue and found that it should be:

'#ajax' => [
  'url' => views_ui_build_form_url($form_state),
]

Instead of:

'#ajax' => [
  'path' => views_ui_build_form_url($form_state),
]

So it is all about 'path' vs 'url'.