Easy Forms: stack trace when editing/adding fieldset tag in a form (BRANCH:hippo-cms-7.9)

When trying to edit or add "fieldset tag" component to an existing form, stack trace is logged [1]

Steps to reproduce:
1. go to CMS, and edit the following document: [/content/documents/easyforms/common/form]
2. add the fieldset tag just before the TextField
3. set the fieldset tag as start tag
4. save the content

Expected: content is saved and no stack trace
Current behavior: content is not save and stack trace occurred [1]