Facing an error after upgrading Civicrm version from 5.78.3 to 6.5.0

In my Backdropcms site, I have recently upgraded the Civicrm version from 5.78.3 to 6.5.0 (latest version).

Besides Civicrm I have other Civicrm related module in my site. These are the modules I have in the site installed - 

1. CiviCRM Theme
2. CiviMember Roles Sync
3. CiviCRM Cron
4. Webform CiviCRM Integration

and some other custom modules for custom functionality. 

I have followed this doc and upgraded the Civicrm. But After the upgrade when i cleared cache and re-enabled the modules, then I got this error and the site is now unreachable.

Error: Call to undefined function civicrm_initialize() in civicrm_member_roles_menu() (line 26 of /civicrm/backdrop/modules/civicrm_member_roles/civicrm_member_roles.module).
 

Anyone have any idea how can i solve this error?