Revisions cannot be viewed - fatal error in code

Today I tried to view the revisions stored for one of my basic pages. When I went to the page and clicked on the Revisions tab, there were many to choose from.

But when I clicked on any of them (date and time), I did not get the page showing Administration menus at the top of the screen and the contents of the page. Instead I got a menu tree of the Admin panel like this:

Skip to main content

Toolbar items

Back to site
Manage

Administration menu

Tools
Index
Flush all caches
Flush CSS and Javascript
Flush plugins cache
Flush render cache
Flush routing and links cache
Flush static cache
Run cron
Run updates
Drupal.org
Change records for Drupal core
D8 API documentation
Logout
Content
Add content
Article
Basic page
Blog post
Entity browser test

etc . . .

At the very bottom of the screen it ended like this:

User account actions

View profile
Edit profile
Log out

Fatal error: Call to a member function bundle() on string in /home/xxxxxx/uuu.yyyyyy.ccc/themes/d8w3css/drupal8_w3css_theme.theme on line 244

where "xxxxxx" above is the name of my account on my hosting server and "uuu.yyyyyy.ccc" is the domain name of the site I am creating. (had to fake out the domain name here so that this form doesn't try to make it a link and break up the full file address location above)

BTW, line 244 in that file is:

if ($content_type = $node->bundle()) {

I tested all of the content - pages and articles - and they all do the same thing.

I have another site I was starting to develop using another theme, and all of the revisions are able to be viewed OK there. So I don't think it is anything I have done.