/',
			$link,
			$item_output,
			1 // Limit.
		);

		// Replace closing </a> with </button>.
		$item_output = preg_replace(
			'#</a>#i',
			'</button>',
			$item_output,
			1 // Limit.
		);

	} elseif ( in_array( 'menu-item-has-children', $item->classes, true ) ) {

		// Add SVG icon to parent items.
		$icon = twentynineteen_get_icon_svg( 'keyboard_arrow_down', 24 );

		$item_output .= sprintf(
			'<button class="submenu-expand" tabindex="-1">%s</button>',
			$icon
		);
	}

	return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'twentynineteen_add_dropdown_icons', 10, 4 );
<?xml version="1.0"?>
<oembed><code>rest_missing_callback_param</code><message>Missing parameter(s): url</message><data><status>400</status><params><oembed>url</oembed></params></data></oembed>