Remove not used themes not possible anymore

[schi]

In TYPO3 CMS 7.6 it is not possible to remove not used themes. This doen't work anymore:

TCEFORM.tt_content.pi_flexform.yag_pi1.sDefault.settings\.theme.removeItems = default,lightbox

But this is possible:

plugin.tx_yag.settings.themes.default >
plugin.tx_yag.settings.themes.lightbox >

In Flexform:
[ INVALID VALUE ("default") ]

This could be solved by removing line 66 in Flexform.xml

<default>default</default>