After countless hours and lots of banging my head against the desk, I discovered that I was getting this error, because I was using xdebug (which is amazing!) to the breakpoint at a low level-level function, namely Magento\Ui\TemplateEngine\Xhtml\Result::__toString() in order to test.
Apparently, in some way, the output from the debugger was actually throwing an error which caused the __toString method to yell at me.
I was going crazy because the error would only show itself when I had the debugger switched on and thought maybe it had something to do with the AJAX call in populating the UI Listing. Switching it off seemed to make it work, so it's worth giving it a shot. I'm not too sure how the breakpoint functionality of xdebug (in IntelliJ products specifically... possibly) works, other than you are able to invoke a breakpoint with the line xdebug_break(). It's very likely that having a breakpoint in the __toString method is just a silly thing to do in any case.
Guess we can't debug everything dynamically yet... One day!
I really hope this helps someone else.
