When adding nested subforms in Joomla 4.0.0-alpha3 and adding multiple rows of the most-inner subform, the add/remove buttons of the more outer subforms are not clickable anymore.

Steps to reproduce the issue

1. Install Joomla 4.0.0-alpha3
2. Edit the file plugins/fields/checkboxes/params/checkboxes.xml and add the following after line 26:

<field
        name="subformlevel2"
        type="subform"
        label="PLG_FIELDS_CHECKBOXES_PARAMS_OPTIONS_LABEL"
        layout="joomla.form.field.subform.repeatable-table"
        icon="list"
        multiple="true"
>
        <form hidden="true" name="list_templates_modal" repeat="true">
                <field
                        name="name_lvl2"
                        type="text"
                        label="PLG_FIELDS_CHECKBOXES_PARAMS_OPTIONS_NAME_LABEL"
                        size="30"
                />
                <field
                        name="subformlevel3"
                        type="subform"
                        label="PLG_FIELDS_CHECKBOXES_PARAMS_OPTIONS_LABEL"
                        layout="joomla.form.field.subform.repeatable-table"
                        icon="list"
                        multiple="true"
                >
                        <form hidden="true" name="list_templates_modal" repeat="true">
                                <field
                                        name="name_lvl3"
                                        type="text"
                                        label="PLG_FIELDS_CHECKBOXES_PARAMS_OPTIONS_NAME_LABEL"
                                        size="30"
                                />
                        </form>
                </field>
        </form>
</field>

3. In the Joomla installation, go to the Backend, Content -> Fields -> New, change the Type to Checkboxes
4. Add a new row of the Checkbox values, and a new row of the then appearing inner subform, and a new row of the then appearing inner subform, so you have 3 rows of the 3 nested subforms - like in this screenshot:
   
5. Try to click on the add or remove buttons of the outer subforms (not the innermost) - they are not working, there isn't even an onclick-event assigned to them.

Expected result

The outermost add/remove buttons should work, they should add a new row of their respective subform.

Actual result

The buttons do nothing, there isn't even an onlick event assigned to them.

System information (as much as possible)

Tested on Joomla 4.0.0-alpha3 using Firefox 62.0 on Linux.

Additional comments

I noticed this problem when testing this functionality in Joomla 4 regarding PR #17552 (re @Fedik )