Error 500 on my vps server, but when i refresh its just running normally

i dont know why i get error like this, so when my server is on production my server usually get an error, but when i refresh its just going well, but i get this error in many times, i've tried everything but its still like that, whats the problem, and when i try to check it the error is like this

A PHP Error was encountered

Severity: Warning
Message: Attempt to read property "db" on null
Filename: Session/Session.php
Line Number: 812
Backtrace:
File: /var/www/html/zonaapoteker/application/controllers/Home.php
Line: 18
Function: set_userdata
File: /var/www/html/zonaapoteker/index.php
Line: 307
Function: require_once
An uncaught Exception was encountered
Type: Error
Message: Call to a member function empty_table() on null
Filename: /var/www/html/zonaapoteker/system/libraries/Session/Session.php
Line Number: 812
Backtrace:
File: /var/www/html/zonaapoteker/application/controllers/Home.php
Line: 18
Function: set_userdata
File: /var/www/html/zonaapoteker/index.php
Line: 307
Function: require_once