Theme kills admin access if activated before WP REST API plugin

Should have noted this one earlier when it happened, but I just thought of it again - I failed to activate the WP REST API plugin before activating the theme and then couldn't access the admin to correct it without removing the theme folder to deactivate it. There was an error message but since I didn't record it at the time I can't grab it for you right now, although it should be pretty easy to recreate - it was about a missing function that the plugin provides, I believe.

Seems like a check to see if the plugin is active would be good.

(Sorry to drop issues that I can't immediately help with!)

Reproduce the failure by activating the theme before the WP REST API plugin and record the resulting missing-function error. Trace the theme's activation entry point and verify that activating it without the plugin leaves the WordPress admin accessible; the issue does not name a file or test to run.

Written by the indexing model from the issue text.