PHP warning – “attempt to read property “title” on null”

Hi, I am working on my website again after a pause of months. I find something strange the last couple of days.

When I click: Appearance –> extra CSS
For a second I see a blank screen with this message. And then the Extra CSS page appears.

“Warning: Attempt to read property “title” on null in /customers/9/6/d/jeroengotz.nl/httpd.www/wp-includes/class-wp-customize-nav-menus.php on line 523 Warning: Cannot modify header information – headers already sent by (output started at /customers/9/6/d/jeroengotz.nl/httpd.www/wp-includes/class-wp-customize-nav-menus.php:523) in /customers/9/6/d/jeroengotz.nl/httpd.www/wp-admin/customize.php on line 126 ”

I tried to figure it out but it goes beyond my knowledge.
I deleted all my navigation bars and rebuilt them, it did not help…
Is it a problem in WordPress self? Does anyone know a sollution so it works properly again?

Thank you!
Have a nice day.

Jeroen

 

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