Thank you. With the above code edit applied, I no longer run into the reported exception when trying to preview the changes.
The 'no changes' preview might be due legacy MediaWiki syntax that I must have missed on some old wiki pages. For example, I have a wiki page from 2011 which is one of the first results in "Reviewing Prepared Results" for the search term [table]. That wiki page does contain a couple of tables however all of them use {| |} MediaWiki table syntax instead of [table][/table] - the page's last revision was before moving to VaultWiki. I do intend to go through these pages with the goal to disable the legacy MediaWiki support at the time of upgrading to XF2.1