Fix error when using multivalued fields in aggregates

Target Platform: LTS 2017 (9.10)
Marketplace Packages: JSF UI

Steps to reproduce:

1. Create a new Content View feature.
2. On the "Search Layout" section, drag and drop a "Terms" aggregate widget.

3. Select the "system" schema and the "tag[]" field.

4. Click "Ok" and notice that nothing really changed (except the dialog backdrop). Clicking "Ok" again will resume the editing as expected.

Notes:

• This is happening because when the "Ok" button is clicked the first time, an error dialog shows up behind the current dialog. This can be verified by hiding the "Edit widget field binding" dialog using the browser dev tools.

• I could only verify this issue with multivalued fields. Other examples are "publish:sections", "mail:recipients" and "mail:cc_recipients".

• Only the "Terms" widget was tested but it's possible that the issue is verifiable with aggregate widgets.

• Not verifiable when adding a "Terms" aggregate widget to a Page Provider.

• The issue doesn't seem to have any serious impact on the generated configuration.