TYPO3 Core

When defining custom content elements, up to the latest version of TYPO3 8 this used to work:

mod.wizards.newContentElement.wizardItems {
    theme.header = Theme
    // or:
    // theme.header = LLL:EXT:theme/Resources/Private...
    theme.elements {
        theme_intro: {...}
    }
}

In TYPO3 9.5.5 the "header" is not recognized anymore. The error is thrown at line 399 in

/typo3/sysext/backend/Classes/Controller/ContentElement/NewContentElementController.php

where JavaScript complains with

rawurlencode() expects parameter 1 to be string, null given

This leads to the problem that the modal for inserting content elements does not show any content elements, but keeps showing the loading spinner.

The included screenshot (xdebug) shows clearly that each item group has its "root" term, defined by the "header" attribute. "elements" are the result of concatenation, as it seems. In any case, "theme" is not recognized, because "header" gets lost somewhere on the way...

Given the fact that this happens in the context of an update from TYPO3 8 to 9, none of the original scripts that worked in v8, changed. However, it would be great, if there is anybody out there, who could (dis)confirm this problem. My own setups (also on new TYPO3 9 installations) all fail.

For reference: The shipped "dummy placeholders for item groups" are defined in:
/typo3/sysext/backend/Configuration/TSconfig/Page/Mod/Wizards/NewContentElement.tsconfig

PS: I assigned "TypoScript" as category, since the failing starts there. But maybe another category wold be more appropriate?