Error in DigitalOcean DNS extension in Plesk after updating it to version 1.3.9: Call to a member function get HeaderLine() on null

Symptoms

After updating the DigitalOcean DNS extension in Plesk to version 1.3.9, opening it or doing authorization in it both via OAuth2 or API token fails:

500 Error
Call to a member function get HeaderLine() on null

Cause

Bug in DigitalOcean DNS extension EXTPLESK-4579 which is fixed in version 1.3.10 of the extension.

Resolution

1. (Only on Plesk 18.0.52) Apply the fix from the article https://support.plesk.com/hc/en-us/articles/14166304366999.

2. Update the extension DigitalOcean DNS to version 1.3.10 or newer.