Steps to reproduce the issue

Create a module / plugin for custom fields that contain a subform-field:

<fieldset name="basic">
	<field
		type="subform"
		name="applications_forest"
		formsource="modules/mod_applications/forms/application.xml"
		multiple="true"
		min="1"
		max="99"
		label="Application" />

..

Inside the subform-xml, add an editor-field:

<fieldset name="application" label="Application">
            <field
                name="application_title"
                type="editor"
                label="Title"
                filter="safehtml" />

..

Install and add a new row to the subform field.

Expected result

By adding new rows to the subform-field, there should show a editor in each row.

Actual result

Editor does not show up, javascript-warning about an empty string being passed to getElementById().
Editor shows only after saving the module/article

System information (as much as possible)

Client: Firefox (54) / OSX 10.10
Server: Php 5.6
Joomla: tried with Joomla 3.6.4 and Joomla 3.7.4
Editor: Tried with JCE (latest) and TinyMCE

Additional comments