Missing mandatory document initialized in the demo package

Reproduction steps:

• Open CKEditor.EditSheet
• Open the object editor and check the content of the UIExtensionClass xobject
• Edit the page, change the title and save and view
• Check the diff between 1.1 and 2.1
• Rollback to 1.1
• Check the diff between 2.1 and 3.1
• Open the object editor and check the content of the UIExtensionClass xobject

Expected result:

• first and last checks of the UIExtensionClass xobject shows that the xobject fields are properly filled
• when checking the diff, only the change on the title should be displayed

Obtained result:

• first check shows that the UIExtensionClass xobject fields are filled, but all values are missing at last check
• when checking the first diff the properties look added to the xobject
• when checking second diff the properties look removed

Apparently the bug doesn't reproduce when creating a page containing an UIExtensionClass, it seems only related to imported page. Also so far the bug has been reproduced on both pages with an UIExtensionClass and a PanelClass.

The regression seems to appear first in 14.9, but it seems it doesn't impact 14.4.6.