error: constructor method deprecated since 4.3

After updating to WordPress 4.3, my website (and the admin interface) is unreliable, often not loading at all. I found a closed topic on this forum about this issue, but it’s closed and didn’t help me.

I get this notice:
The called constructor method for WP_Widget is deprecated since version 4.3.0! Use __construct() instead. in /homepages/39/d230859104/htdocs/cado/wp-includes/functions.php on line 3457

I’ve located the above file, but line 3457 is part of function _deprecated_constructor( $class, $version ), which reports the error.

I find “WP_Widget” just twice in functions.php code:

add_action( '_admin_menu', 'wp_widgets_add_menu' );

and

function wp_widgets_add_menu() {
	global $submenu;

	if ( ! current_theme_supports( 'widgets' ) )
		return;

	$submenu['themes.php'][7] = array( __( 'Widgets' ), 'edit_theme_options', 'widgets.php' );
	ksort( $submenu['themes.php'], SORT_NUMERIC );
}

but I don’t know what to do with __construct()

the site is capedownwinders.org, I’m using the Mantra theme with a child modification – all worked well prior to the WP update.

Any help much appreciated!