New Issue, Cannot Modify Header Information Warning

Hello I have read a few similar scenarios.

I have the latest version of WordPress 5.0.2
After the previous WP update I had intermittent warnings, including these:

Notice: Trying to get property of non-object in /home/xyz3pys8w/public_html/wp-includes/nav-menu.php on line 780

Trying to get property of non-object in /home/xyz3pys8w//public_html/wp-includes/nav-menu.php on line 784
Pages

Last night they appeared after a backup and theme update.

Trouble shooting I have completed:

1. Deactivated all plugins one by one to test if conflict – none
2. Activated a different WP theme – issue remained

3. Checked the wp-includes, nav-menu/php lines of code 780 and 784 to see if find anything but cannot. I am not willing to touch the PHP code

4. Contacted theme developer, Sober Theme, no reply yet

5. Contacted Hostgator: Chats dropped 5 times. No resolution except to tell me it is a WP core issue in wp-includes folder and to contact developer.

6. Did a Wordfence Scan ( Have Premium Paid Account) results – no issues found.

Yet, I still have these warnings appearing at the footer of the sites homepage on front end. Can’t think of what else I can do, except try to get hold of developer. Confused by Hostgator says WP core issue, so is that the theme developer’s problem?

Appreciate any suggestions or help.

Additional notes –

This is what Hostgator tech basically replied with:

The error that your site is throwing is located in the wp-includes folder which is the folder that contains the core WordPress files.

I do apologize but you need to contact a developer to fix that since those are core files that WordPress needed in order to load properly.

Normally, WordPress is having an issue due to themes and plugins but that is located in different folder. For this one, it is the core files that is having an issue.
By the way, what was the last thing you did prior to the error?

Those core files are delicate files that only a professional web developer who know how to program in PHP can resolved.

Normally, this error to core files are caused by some bad plugins or theme issues. When core files are affected, it is best to contact a web developer to fix the issue or it will cause issue if we go willy nilly on the code. I hope you understand.