AdminOnly causes a Notice: Undefined offset in wordpress-develop vars.

[hellofromtonya]

After fixing the AdminOnly test group, we now get the following notice:

Notice: Undefined offset: 1 in /Users/hellofromtonya/Local Sites/wordpress-develop/src/wp-includes/vars.php on line 32

The problem is flagged here https://github.com/WordPress/wordpress-develop/blob/master/src/wp-includes/vars.php#L23-L32

// On which page are we?
if ( is_admin() ) {
	// wp-admin pages are checked more carefully.
	if ( is_network_admin() ) {
		preg_match( '#/wp-admin/network/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
	} elseif ( is_user_admin() ) {
		preg_match( '#/wp-admin/user/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
	} else {
		preg_match( '#/wp-admin/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
	}
	$pagenow = $self_matches[1];

It may also be linked to another problem we see Rocket when running the AdminOnly test suite:

Test code or tested code did not (only) close its own output buffers