How to get id of selected entity in an "entity_autocomplete" form element?
Seemingly I managed to find the solution.

The event for the autocomplete element should be "autocompleteclose" instead "autocompleteselect". 
And in this case it's possible to get entity_id via 

$entity_id = $form_state-&gt;getValue('autocomplete_places');


if the list of values was closed without selected element then $form_state->getValue('autocomplete_places') is null.