TypeError: Drupal.theme.bootstrapModal is not a function

I had a little issue on my site with a bootstrap theme. Instead of showing up the "insert xyz" entity popup, I got the following console error:
TypeError: Drupal.theme.bootstrapModal is not a function

I had to patch the file "entity_embed.routing.yml" and added the following two lines of code at the end of the file:

  options:
    _theme: ajax_base_page

After this little tweak, the entity browser opens nicely in a modal window.
Hope this helps somebody.