Joomla website, fix error: 0 Call to a member function q() on null

I need you to fix the error on this website:
https://www.offirent.be/diensten/diensten-offirent

"0 Call to a member function q() on null"

PHP Version 7.2.34
Joomla! 3.10.2 Stable [ Daraja ] 12-September-2021 11:01 GMT

Second fix the menu structure:
https://www.offirent.be/
https://prnt.sc/1wtuntz

If it is possible to do this fix today.

Later we need also a new template what is not bloated, but fast loading.

Thank you in advance for you're time.