register_rest_route is being called incorrectly

[richtabor]

Describe the bug:

The following notice (screenshot) is present within WP 5.7.

Expected behavior:

No errors are present.

Screenshots:

Isolating the problem:

• [x] This bug happens with no other plugins activated
• [x] This bug happens with the default Twenty Twenty WordPress theme active
• [x] This bug happens without the Gutenberg plugin active
• [x] I can reproduce this bug consistently using the steps above

WordPress version:

5.7

Gutenberg version:

n/a