WP 5.0.3 Call to a member function filter_post_class() on null

[sebaturen]

Hi!, I update my wordpress version to 5.0.3 and when try save or public a new page, the wordpres say "Fail to update" and in error.log file I can find:

[15-Feb-2019 19:28:56 UTC] PHP Fatal error:  Call to a member function filter_post_class() on null in /<wordpress folder>/wp-content/themes/wonderflux/functions.php on line 870

thx for your help :D

*I try install clear wordpress and wonderflux (2.51) but I have same problem

[sebaturen]

I see not is critical, I get a error in WP and in error.log but page is update anyway only title...

[sebaturen]

I update to developer version (2.6) and I can edit content and save, but if I edit the page template, the WP say "Fail to update" but I can't get WP error in any file