@thefactory
Sorry for the issue you are having. This issue is caused by a bad plugin or theme most of the time. Are you running a WooTheme? If so, change your theme to the twenty sixteen theme and see if the issue is gone.
If the issue if you theme, please contact the theme developer.
Please refer to http://support.template-help.com/index.php?/Tickets/Submit for submitting a ticket to them about the issue you are having.
I submitted a ticket, however I feel this issue is more WordPress related due to numerous other threads created with the same issue.
You are more than welcome to post here as well. The other issues that are popping up here are theme related. Refer to the last reply on page 1 here: https://ww.wp.xz.cn/support/topic/warning-after-the-upgrade-to-wp-44?replies=12
All the other issues seem to be this or related. Basically the theme is try to map array is null or incorrect values.
Update… Error has to do with Events Manager Pro plugin. Once I enabled tags and enabled categories in the Events Manager Settings. This error went away. I have posted a support ticket on their website.