Dynamic rows JS error in optimizer form with category limitation and no search terms

[rbayet]

When an optimizer is configured for both catalog and search containers and category limitations are configured, there is a javascript error produced when the form is re-opened after the form has been saved while the search terms limitation panel was left opened without any terms selected.

Preconditions

Magento Version : 2.3.0 EE, CE

ElasticSuite Version : 2.8.x-dev

Environment : Developer and Production

Third party modules : N/A

Steps to reproduce

1. Create a new optimizer configured to be available in all three scopes, save and continue
2. Add a category limitation, save and continue OR save and edit the optimizer again
3. Start adding a search terms limitation, but do not select any search terms, so the " Catalog Product Search / Apply to" select is set on "Selected search terms" but with an empty selection
4. Save and continue OR save and edit the optimizer again

Expected result

1. The optimizer edit form page should load correctly without any error

Actual result

1. A javascript error is thrown and visible in console "TypeError: obj is null" on line 186 of dynamic-rows-grids.js