Not able to ADD pages in Fixer Plugin

Hello, I have fixer theme installed. I was updating the layout to get to make it mobile friendly but the new pages I added stopped loading. I see 'white screen of death' when loading below pages:
I tried adding new page but same issue:

Some old pages are loading like

I enabled debugging mode so we can see the error, and it shows:

Notice: Object of class WP_Error could not be converted to int in /home/dpartner/public_html/tpartners/wp-includes/nav-menu.php on line 355 Notice:

I googled the issue and found the below, link:

But still could not resolve. Please advice. I also granted support access to WPMU to the site.

Thank you.