No notification number in the bubble if `Show only participating notifications` is enabled

[darkred]

(follow-up to https://github.com/tanmayrajani/notifications-preview-github/issues/89#issuecomment-618091190)

(Tested extension 20.4.22 in a fresh Chrome 81/Firefox 75 profile)

if I enable Show only participating notifications then the notification number in the bubble is no longer displayed, even if Show notification number in blue bubble remains enabled.

The following error is repeatedly shown in console :

(github-notifications-preview.js:386) 
Uncaught (in promise) TypeError: Cannot read property 'textContent' of null 
     at $i4sc$var$updateUnreadCount 

Screenshot comparison having Show only participating notifications :

Disabled --> notification number (4) is displayed ok

Enabled --> notification number (2) is missing