Widgets screen: PHP notices with the legacy Recent Posts widget and possibly other legacy widgets

[afercia]

Describe the bug
While testing the new Widgets screen, I noticed a blank space at the top of the admin menu:

this space is added by the core php-error CSS class set on the body when some PHP notice/warning/error is detected. Its purpose is to make the notice/warning/error text fully visible. However, there was no output so this was a bit difficult to debug.

Turns out it happens when a Recent Posts widget is already in use.

To reproduce

• on 5.6. trunk, active theme: Twenty Twenty
• make sure define( 'WP_DEBUG', true ); is set in your wp-config.php file
• deactivate the Gutenberg plugin
• go to the legacy Widgets page and add a Recent Posts widget in the Footer TinyMCE formatting improvements #1 area
• save
• navigate away from the Widgets page
• activate the Gutenberg plugin
• go to the new Widgets screen
• notice the space at the top of the admin menu
• enable the Query Monitor plugin
• see it lists 4 "Undefined index" notices related to the arguments before_widget, before_title, after_title, after_widget

Screenshot:

I tested only with the Recent Posts widget but likely these notices are triggered also with other legacy widgets.

Expected behavior
To not have PHP notices.

Editor version (please complete the following information):

• WordPress version: 5.6 trunk
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? plugin
• If the Gutenberg plugin is installed, which version is it? 9.1.1

[afercia]

Props to @fklein-lu for reminding me to use Query Monitor and to @johnbillion for making it 😄

[johnbillion]

Possibly https://core.trac.wordpress.org/ticket/51383 ? (Screenshot in https://core.trac.wordpress.org/ticket/51073)

[johnbillion]

Hmm no because those aren't suppressed errors. Same result, different cause.

[talldan]

I was also seeing this issue previously, but not anymore. Anyone else?

[noisysocks]

I still see this whitespace locally. Not sure if it matters, but I'm running PHP 8.

[kevin940726]

I'm not seeing this error on WordPress 5.6, Gutenberg 9.7.0, and following all the above reproduction steps. 🤔

[noisysocks]

I am no longer able to reproduce this in trunk. Thanks for the detailed reproduction steps @afercia.