I also experienced the same fault immediately after my Kunena 5.1 update.

I substituted the two files mentioned on these two pages.

https://stackideas.com/forums/kunena-plugin#reply-405126

https://stackideas.com/forums/kunena-easysocial-app-error-on-new-kunena-5-1-call-to-undefined-method-kunenaforumtopic-authoris

The problem was corrected.

I noticed that both previous posts were closed. For others that might run into this problem..... I thought a repeat post might help.