Steps to reproduce the issue

on 4.0-dev install some data, index it and use the smart search in frontend (through a menu item or module)

Expected result

Suggestions proposed after entering a few letters

Actual result

No suggestions

Hints

1. use Joomla\String\StringHelper;
   is missing in
   /components/com_finder/src/Model/SuggestionsModel.php

2. After correcting this, Console error
   Uncaught SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data [finder.js:34:31]

and Warning
[26-Dec-2020 06:43:33 UTC] PHP Warning: Invalid argument supplied for foreach() in .../components/com_finder/src/Model/SuggestionsModel.php on line 51

Looks like this last one happens because $items is false.

@Hackwar