Gutenberg is unintentionally incompatible with WordPress 5.2.x. #20484
Description
Describe the bug
Introduced in #17151. Quoting from #17151 (comment)
The introduction of gutenberg_extend_settings_image_dimensions breaks Gutenberg in WordPress 5.2.x, due to the call to wp_get_registered_image_subsizes, a function introduced in 5.3.0.
The Gutenberg plugin requires WordPress 5.2.0+. We should either bump that to 5.3.0, or guard against missing wp_get_registered_image_subsizes in lib/client-assets.php.