[TYPO3] $GLOBALS['TCA'] seems to be empty on static function call in FE-Extension

[Johannes Konert]

Hello members,
it keeps me busy since a week:
I have a small extension manipulating the available pageTypes and 
serving the registered pageTypes to interested callers.
One function returns all Ids of the available pageTypes by doing:
function submenugenerator() {
   global $GLOBALS;
   t3lib_div::loadTCA('pages');
   foreach 
($GLOBALS['TCA']['pages']['columns']['doktype']['config']['items'] as 
$item){
             // search for doktype-keys and specific pagetypes and find 
the page-uids in table 'pages' for them
   }
   return $pageIDsMenuArray;
}

I include this by TypoScript:
includeLibs.tx_myext = EXT:myext/class.tx_myext.php
temp.mymenu = HMENU
temp.mymenu {
   ..
   special.value.preUserFunc = tx_myext->submenugenerator
   special.if.isTrue.data.preUserFunc = tx_myext->submenugenerator
  1=TMENU
  ..
}

So: This results in a nice menu in Frontend and works as it should, but 
only if the ADMIN-User has a BE-Session at the same time. If the Admin 
is not in the Backend, the foreach-Loop is not executing and giving a 
PHP-warning: Warning: Invalid argument supplied for foreach() in ...

Why is $GLOBALS['TCA'] empty if my FE-Extension-Function is called by 
TypoScript? (And why is it not empty if the Admin is in the BE the same 
time?)

Thanks for any help how to get the $GLOBALS['TCA'] have some content in 
all cirumstances and not just with the admin using the backend at the 
same time..I do not want to click around in backend for the rest of my 
life to make the menu work ;-)
Regards to all of you
Johannes