Error thrown

Cannot unset string offsets

PHP Warning: Declaration of titan_submenu::start_lvl(&$output, $depth) should be compatible with Walker_Nav_Menu::start_lvl(&$output, $depth = 0, $args = Array) in D:\Data\Web\stirkmedicalgroup\wp-content\themes\titan\functions.php on line 460 PHP Warning: Illegal string offset 'echo' in D:\Data\Web\stirkmedicalgroup\wp-content\plugins\html-sitemap\html-sitemap.php on line 60 PHP Warning: Illegal string offset 'title_li' in D:\Data\Web\stirkmedicalgroup\wp-content\plugins\html-sitemap\html-sitemap.php on line 61 PHP Warning: Cannot assign an empty string to a string offset in D:\Data\Web\stirkmedicalgroup\wp-content\plugins\html-sitemap\html-sitemap.php on line 61