Website breaks when I change to PHP 7.4 from PHP 7.0 . Help please:)

Cant upgrade from PHP 7.0 to 7.4 without website breaking.HELP please:)

I am using the latest version of JOOMLA 3.9.26 I am using this template called ELOS purchased from themeforest. https://themeforest.net/item/elos-responsive-multipurpose-joomla-theme/8643058

I have all plugins updated. When I change my PHP setting to 7.4 on my hosting panel I get the message below:

0 - Using $this when not in object context

My website only works when I revert it back to PHP 7.0

I have attached a screenshot and listing of errors when I change it to 7.4. My website is www.connormaguire.com

Any help greatly appreciated. Thanks:)

*[0 - Using $this when not in object context You may not be able to visit this page because of: an out-of-date bookmark/favourite a search engine that has an out-of-date listing for this site a mistyped address you have no access to this page The requested resource was not found. An error has occurred while processing your request. Please try one of the following pages: Home Page If difficulties persist, please contact the System Administrator of this site and report the error below.. Using $this when not in object context Call stack

Function Location

1 () JROOT/libraries/src/Application/CMSApplication.php:390 2 Joomla\CMS\Application\CMSApplication::getMenu() JROOT/libraries/src/Application/SiteApplication.php:275 3 Joomla\CMS\Application\SiteApplication::getMenu() JROOT/templates/elos/preset/index_default.php:13 4 require() JROOT/templates/elos/index.php:15 5 require() JROOT/libraries/src/Document/HtmlDocument.php:668 6 Joomla\CMS\Document\HtmlDocument->_loadTemplate() JROOT/libraries/src/Document/HtmlDocument.php:730 7 Joomla\CMS\Document\HtmlDocument->_fetchTemplate() JROOT/libraries/src/Document/HtmlDocument.php:545 8 Joomla\CMS\Document\HtmlDocument->parse() JROOT/libraries/src/Application/CMSApplication.php:1076 9 Joomla\CMS\Application\CMSApplication->render() JROOT/libraries/src/Application/SiteApplication.php:778 10 Joomla\CMS\Application\SiteApplication->render() JROOT/libraries/src/Application/CMSApplication.php:209 11 Joomla\CMS\Application\CMSApplication->execute() JROOT/index.php:51]1*