TYPO3 Core

It is not possible to add an element of type inline inside an element of type inline in the page view.
An "error 500" message is showing up. The logge error message is:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1446996319: Configuration retrieved from FlexForm is incomplete or not of type "inline". | UnexpectedValueException thrown in file /var/www/html/typo3_src-7.6.21-bi/typo3/sysext/backend/Classes/Controller/FormInlineAjaxController.php in line 912.

The same error occurs while trying to expand a collapsed inline element inside an other inline element.

The error is only present in the page view. If I edit the "upper" inline element in list view, there is no error.

It's the same problem described in #71436 and the patch in https://forge.typo3.org/issues/71436#note-49 works when manually applied (see comment of Bart Lammers).

Update: See the working patch in comment #7

More detailed description to reproduce the error:
There is a plugin where I can add elements of type "inline" (in backend). These elements are the parent elements and contain a FlexForm definition. In that FlexForm area I can add some child elements (also of type "inline").
So when I edit my plugin and add some parent element and I try to add a child element, the error occurs. If I save my plugin (with the added parent element) and change to the list view, I am able to edit the parent element directly and add a child element.

The error did not exists in TYPO3 6.2.