Deprecated function breaks website

The Child Theme Generator plugin has been working without errors for months; but today started causing this error, preventing anyone from viewing or logging in:

Deprecated: Creation of dynamic property Ch_Th_Gen::$functions is deprecated in /home/customer/www/staging3.focosustainability.org/public_html/wp-content/plugins/child-theme-generator/includes/class-child-theme-generator.php on line 130

How can I fix this?

I can log into the hosting company, Siteground, to access the file manager; but cannot log into this staging copy of the website.

This type of error is caused by php outputting some text or cookies before it should; but looking at the code indicated by the above error message does not show what is causing the problem.

Any help appreciated….

–Rick Casey

The page I need help with: [log in to see the link]