getForm returns null?!

i know, i know the question is quite simple but i encounter the following problem when working with a Dynpage:

trying to add components to a form in the doProcessBeforeOutput() method, i get a NullpointerException as getForm() return null.

(i also did create a document -getPageContext().createDocument()- and added a form, but the ui-components added didn't show up )

so are there some additional steps i have to follow, that are not mentioned in the documentation?

its weired as no one seems to ever have encountered the problem : /.

thanks in advance,

patrick