[Solution] Template error: [E_USER_WARNING] The isMemberOf method cannot be called for a non-object (NULL)

[Linda]

I would like to share my solution with you.

This error occurs when you have deleted a user or a plugin, this leaves traces in your database. Visit your database administration, search for "log" and.
Empty (do not delete!!!!) the database entries.

The server log will also determine where the deleted user's post is located. Remove it.

 

[Thanos Crax]

Hmmm maybe this can be applicable to a similar issue of mine
https://enxf.net/threads/errorexception-template-error-e_user_warning-cannot-call-method-isonline-on-a-non-object-null-src-xf-template-templater-php-1176.8300/