Sansei Demo content Issue PHP version issue

After installing

more specifically this theme started giving me such errors

Warning: get_class() expects parameter 1 to be object, null given in /home/customer/www/ on line 1031 and one more error

after I downgraded the PHP version of my hosting it solved but my wocommerce wont run flawlessly if I use downgraded PHP,

after I install the demo designs it seems like many things are missing from there i have installed all plugins all files.