unload class dotnetnuke.PortalModuleControl,Why?

when to open Editxxx.ascx of dotnetnuke in VS.NET,prompt a hint like that:
"unload class dotnetnuke.PortalModuleControl"

in Object explore,i found this PortalModuleControl which defined in

dotnetnuke\components\DesktopControls.vb

when i made a new module which inherit this PortalModuleControl,there're
some errors related to this control.

i don't know how to solve this problem.