There is a missing validation into the function social_profile_get_default_image:
Error: Call to a member function id() on null in social_profile_get_default_image() (line 792 of /var/www/html/web/profiles/contrib/open_social/modules/social_features/social_profile/social_profile.module)
As I see, by just validating if file exist should fix issue. Patch in the way.
Closed: duplicate
11.3
Code (back-end)
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
These two tickets are related but not duplicates. In #3271758, the file is validated, and a call to social_profile_get_default_image() is prevented. However, social_profile_get_default_image() is called in multiple places, e.g., by EmailTokenServices::getUserPreview(). As wassper correctly assumed, a check is needed to see if the file exists in social_profile_get_default_image().
Here is a new approach for open_social 12.