How to fix logs on Joomla after updating php to version 7.1?

Question

Joomla

After updating php to 7.1, instead of the site, an error pops up:
Error: 0
Using $this when not in object context
According to the logs, it shows that the errors are in the template in the /templates/template name/php/template.php file
, namely in these lines:

$path = JPATH_THEMES . DS . $this->_document->template;

$this->flashFull = $this->pathFull . DS . 'flash';
        $this->imgFull   = $this->pathFull . DS . 'images';
        $this->cssFull   = $this->pathFull . DS . 'css';
        $this->jsFull    = $this->pathFull . DS . 'js';