"Undefined Variable" Error after Site Migration

i'm Running CI 3 with PHP 7.4. and i have not encountered any error.

seems like the error "Call to a member function result() on null" is from a database result.

my wild guess since you have upgraded to PHP 7.4 there is a chance that mysql was also upgraded. 

If it turns out to be mysql8 or above i think its an error on the caching_sha2_password

you can test by uploading a file on your server which contains a simple database connection.

if not please disregard my reply.

Thanks