Opting out of the widgets-block-editor theme support crashes the editor serverside

[draganescu]

Since merging #29649 we introduced a show_instance_in_rest property to widgets to determine their visibility for the Legacy Widget Block and other consumers.

Because core widgets don't yet have this property we have a function gutenberg_set_show_instance_in_rest_on_core_widgets which as the name says adds this property and sets it to true.

However part of the core widgets we also consider WP_Widget_Block which does not get registered if widgets-block-editor theme support is removed. This results in a fatal error:

Warning: Creating default object from empty value in /lib/widgets.php on line 317
wp-includes/class-wp-hook.php:292 gutenberg_set_show_instance_in_rest_on_core_widgets(), wp-includes/class-wp-hook.php:316 apply_filters(''), wp-includes/plugin.php:484 do_action(Array), wp-includes/widgets.php:1801 do_action('widgets_init'), wp-includes/class-wp-hook.php:292 wp_widgets_init(), wp-includes/class-wp-hook.php:316 apply_filters(''), wp-includes/plugin.php:484 do_action(Array), wp-settings.php:553 do_action('init'), wp-config.php:7 require('wp-settings.php'), wp-load.php:37 require_once('wp-config.php'), wp-admin/admin.php:34 require_once('wp-load.php'), wp-admin/post-new.php:10 require_once('wp-admin/admin.php')]

A solution (via @fullofcaffeine) is to check if the widget is registered before we try to set its properties. Since unregister_widget exists we should check this anyway until that function is merged and we don't need to loop through all core widgets anymore.