Takai template work fine in php 7.1 version, but in version 7.2 or 7.3 not work. Where is problem? Please help me.

This error:
Notice: Undefined property: Joomla\CMS\Document\ErrorDocument::$params in /home/zagor/public_html/templates/takai/includes/helper.php on line 92
Error: Call to a member function get() on null: Call to undefined function mb_internal_encoding()

I solved the problem
'Mbstring' must be enabled on the server in php options
The site works great