UI is broken when user tries to create a subpage of a subpage linked to a Site Template page

• Type: Bug
• Status: Closed
• Resolution: Fixed
• Affects Version/s: 6.2.0 CE RC5
• Fix Version/s: 6.2.0 CE RC5
• Component/s: Frontend Infrastructure, Site Templates, WCM, WCM > Sites Management
• Labels:
  • QA-KC
  • QA-R
• Story Points: 3
• Fix Priority: 4
• Git Pull Request: https://github.com/brianchandotcom/liferay-portal/pull/14658

1. Create a Site Template (myTemplate) with one page: page1
2. Create a subpage of page1: subpage1
3. Create a new "myTemplate" site (testSite)
4. Go to Admin Site (testSite) : Pages
5. Select the supage1 and try to add a subpage: subpagetest

Result: UI is broken, see attached image.
Expected: User can't add a subpage like this, currently subpages of pages linked to site template pages are not allowed.