QuickLook by the fly generation (RTF)

Hallo NG,

I have some problems to generate a preview for Quicklook.
The example:
http://developer.apple.com/documentation/UserExperience/Conceptual/Qui...ook_Pro
is not working for me. By the way, I also don't understand why the last byte of
the attributed string has to be stripped. But that is not my real problem.
I my code I do nearly the same like in the example, but I get an exception:

#0 0x93cea114 in ___TERMINATING_DUE_TO_UNCAUGHT_EXCEPTION___ 
.... 
#3 0x91a534a3 in -[NSObject(NSKeyValueCoding) valueForUndefinedKey:] 
 
A short description what I do in GeneratePreviewForURL. 
I use NSUnarchiver to get my document from the path. 
(perhaps that is the  problem). 
In my document I have a function -(NSAttributedString*)contentAsRFTD to  
generate a RFTD-String. This is converted to a cfdataref and processed by  
the QLPreviewRequestSetDataRepresentation()function. 
Everything until this function call seems to work fine. I get the document,  
the and Attributed string, but the function quits with this exception.

TIA Florian