Modal needs to be open twice to work

Problem/Motivation

The first time you load a node form and click on a Entity Reference Tree button, its modal is open but no tree is loaded. You need to close that modal and click on the button again so the taxonomy tree is actually loaded.

Steps to reproduce

Load a node form, click on a Entity Reference Tree button. The modal that gets open doesn't load any data.

That happens randomly, more usually where Drupal JS files are served from a slow connection.

Proposed resolution

Ensure that Drupal.attachBehaviors(); is called when the modal is open.