Unable to access domain PHP Settings and Hosting Settings: ERROR: Method Smb_Form_Final_Web_PhpSettings::__toString() must not throw an exception

Symptoms

• The following error appears while pressing Domains > example.com > PHP Settings or Domains > example.com > Hosting Settings on a domain:
  

  PLESK_ERROR: ERROR: Method Smb_Form_Final_Web_PhpSettings::__toString() must not throw an exception, caught Error: Call to a member function getId() on null (php-settings.phtml:0)

• All PHP-FPM handlers are disabled on the server

Cause

This is the Plesk bug with the ID #PPPM-12457, which is planned to be fixed in the next Plesk update release.

Resolution

1. Log into Plesk.
2. Go to Domains > example.com > Apache & Nginx Settings.
3. Tick the option Proxy mode.
4. Scroll down and press the OK button to apply the changes.