Fix Web UI diff after Polymer 3 migration

Differences between two versions of a document are not displayed when using the Compare button.

Steps to reproduce:

• As Administrator, create a document.
• As a different user with ReadWrite permission on the above document, edit the document's Description, Nature and Subjects.
  => Version increments to 0.1+.
• Click on the Compare button to compare the working version with the original document.
  => No differences are displayed below (see attached screenshots)

Errors in console:

Uncaught TypeError: Cannot convert undefined or null to object