After upgrading from 3.8.8 to 3.9.1, I'm not able to create a new menu item in the back-end.

Steps to reproduce the issue

Click on the "New" to create a new menu item for any menu.

Expected result

Should open a UI to create a new menu item.

Actual result

An error has occurred.
Cannot use object of type stdClass as array

System information (as much as possible)

Linux dev2.karjalainen.fi 4.14.77-70.82.amzn1.x86_64 #1 SMP Mon Dec 3 20:01:27 UTC 2018 x86_64
MySql [ 5.5.5-10.0.24-MariaDB ]
PHP 7.0.32
WebServer nginx/1.14.1
WebServer to PHP Interface | fpm-fcgi
Joomla! 3.9.1 Stable [ Amani ] 27-November-2018 15:00 GMT

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