Adding already existing entities via entity_browser / entity_form causes javascript error

Problem/Motivation

• while adding already existing entities via entity_browser in a reference field no error message is shown / instead an JS error is thrown that prevents opening the entity_browser again

Steps to reproduce

• Create custom entity_browser -> Modal with view for selection of content entities
• Create entity reference field in content type with form settings
• Reference items that could be selected in previously created entity_browser
• Widget: Inline Entity form complex (Allow users to add existing nodes.)
• Widget: select created entity browser in the "Entity Browser" select
• Create new content and try to select existing content via Entity browser
• Content is added in inline-entity-form table
• Try adding the same content again via entity_browser
• Current Behaviour: nothing happens -> JS error in console
• Expected Behaviour: show form error, that entity was already added to ief

Error: in console

Proposed resolution

• fix error handling in form and show error, while trying to add an already existing entity

Expected behavior:

I'll attach a patch fixing the issue.