Alright, I copied just the text over, and that appears to have fixed the issue! I'm not seeing any errors come up in the web console, and the UploadWizard is working normally now! Thank you!

In T264965#6584837, @Reedy wrote:

Your error makes it sound like you've taken the whole file from master, which will not work. In REL1_35, the LoadBalancer parameter isn't passed at all. It is in master, due to rMWe9c0f8236c5f: Migrate more User::newFrom* methods to UserFactory. See https://github.com/wikimedia/mediawiki/blob/master/includes/ServiceWiring.php#L1359-L1364

Just tested this out this morning, and now I'm getting this HTTP 500 error:

In T264965#6548229, @matmarex wrote:

Looks like a problem with the definition of mediawiki.messagePoster in core MediaWiki.

That did it! Visual Editor is loading normally from "Edit" and I can switch to it from the source editor, too, without any issues!

OKAY, here goes. It's a clean install, using the web installer.
Setup:

The HTTP 500 error always triggers when clicking on "Edit", and reappears when clicking on "Try Again". When switching from source to visual editor, the HTTP 500 error always triggers at first, then the error message "There is no revision with ID 0" appears when clicking on "Try Again." I can try the other things.

Just set up MW 1.35.0-rc.0 with the bundled extensions, getting the same error. Also, when switching from Source Editor to Visual Editor, I get the same RESTBase server error first, then get "There is no revision with ID 0." after clicking on "Try Again." The revision error keeps repeating after that. I imagine it's related.