Bail out if get_current_screen() returns null.

[hellofromtonya]

Description

In function gutenberg_widgets_customize_load_block_editor_scripts_and_styles() on < WordPress 5.8-RC2, a PHP notice is thrown when get_current_screen() returns null.

PHP Warning:  Attempt to read property "base" on null in /.../wp-content/plugins/gutenberg/lib/widgets-customize.php on line 162

This happens because the instance is used to get the base property as part of the conditional check.

This PR breaks up the conditional checks into guard clauses and then checks if the value returned from get_current_screen(0 is null. If yes, it bails out. If no, then the base property is checked.

How has this been tested?

Local machine => Notice in debug log before fix. No notice after fix.

Environment:

• OS: macOS Big Sur
• WP: 5.7.2
• Browser: Chrome and Firefox

Note: The PHP notice does not happen with WordPress 5.8-RC2.

Screenshots

Types of changes

• Breaks up the conditionals and converts each into a guard clause
• Checks for null and bails out if returned

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[getdave]

Tested with Plugin build of this PR and found no errors in Widgets 🥳

[getdave]

Does this need backport to WP 5.8?

[aristath]

Does this need backport to WP 5.8?

It would be good to backport it, I ran into this issue when trying to patch https://core.trac.wordpress.org/ticket/53429 and had to resort to a somewhat ugly hack (WordPress/wordpress-develop@069d32f)
If this gets backported we'll be able to properly fix the other issues as well 👍

[hellofromtonya]

Does this need backport to WP 5.8?

This particular callback function is not in WP 5.8. The PHP notice is not thrown in 5.8-RC2. That said, would be worth opening a new Trac ticket to investigate the problem you saw @aristath as there may be a hole in the merged implementation that needs a check like this.

[hellofromtonya]

Why not check for the instanceof WP_Screen?

• As this is a plugin, an additional check might be required to ensure the class WP_Screen exists.
• get_current_screen returns null on failure or an instance of WP_Screen on success.

The check here is for the failure state. Using this approach keeps the check positive and simple, while avoiding an additional class_exists check.

[TimothyBJacobs]

FWIW, you can safely do an instanceof check with a class that does not exist., https://3v4l.org/X4n64.