Plugin: Account for null return of get_current_screen

[aduth]

Fixes #14547

This pull request seeks to resolve issues where calling the deprecated functions is_gutenberg_page or gutenberg_init in a non-WP-Admin context may produce fatal errors.

For additional context, see #14547 (comment) .

Additional unguarded references to get_current_screen exist in lib/widgets.php. However, these functions are action handlers for admin_print_styles, at which point it's considered an assumed guarantee that get_current_screen would exist and return a non-null value (the screen is set immediately prior to the actions firing). This is still a loose guarantee, and it may be advisable to consider introducing guarded conditions all the same (e.g. for handling an unprompted plugin's do_action( 'admin_print_styles' );).

Testing instructions:

Place the following file as e.g. is-gutenberg-page.php within wp-content/mu-plugins. There should be no fatal error when visiting the front of the site.

<?php

add_action( 'init', function() {
	is_gutenberg_page();
} );

[youknowriad]

Thanks for the fix.