"Assertion failed in ...CtrlCore\Win32Wnd.cpp,line 528
top->hwnd"
My guess is that CreateWindowEx has failed.
My guts feeling is that this is because U++ is not properly initialized. Initialization creates window classes used with CreateWindowEx and that is what IMO fails here.
This is at the moment unsupported scenario. Anyway, if you want to try, check the definition of GUI_APP_MAIN macro and try to reproduice it with DLL_APP_MAIN (e.g. create something like DLL_GUI_APP_MAIN?)
Keep me informed, it would be nice to support this...