Problems with an MDI application

MEnu handler doesnt have much code .
In the menu I have a login menu pick and when the user logs in
I show a modeless property sheet and they can download data with different properties set.As soon as the data gets downloaded I have to plot a chart in the view.I have a Plot button on the view and it plots fine.
Now the problem is the menu pick has a'New Window' and when thats done it gives me the ASSERT FAILURE.My guess is its not finding the hadle for the child window and my question is how do I force it to create the child Window again.
It goes to MFC code so I was not sure how.
THanks

if(pParentWnd=NULL)
{
CWnd* pMainWnd = AfxGetThread()->m_pMainWnd;
ASSERT(pMainWnd != NULL);
ASSERT_KINDOF(CMDIFrameWnd, pMainWnd);
pParentWnd = (CMDIFrameWnd*)pMainWnd;
}
It fails on the ASSERT_KINDOF as the handler is the dialogs and not he CMDIFRAMEWNDS.I wanted to get the CMDIFRAMEWNDS handler.
THanks

if(pParentWnd==NULL)
{
CWnd* pMainWnd = AfxGetThread()->m_pMainWnd;
ASSERT(pMainWnd != NULL);
ASSERT_KINDOF(CMDIFrameWnd, pMainWnd);
pParentWnd = (CMDIFrameWnd*)pMainWnd;
}
gives an error too.
First time I run the Application it works fine.THen on the menu pick I have a pick to go to a dialog bvox and then when I come back and try
'New Window'
it gives me an ASSERT failure on this.

THANKS A LOT!!IT WORKS!!THANKS AGGAIN.THANKS A LOTTTT.I AM SO HAPPY

Heres the code I have in the onLogin menu handler

CSampleSheet *m_dlgpropertysheet=new CSampleSheet("Sample Application");
ASSERT(m_dlgpropertysheet);
_snprintf(m_dlgpropertysheet->m_szTitle, sizeof(m_dlgpropertysheet->m_szTitle)-1, "%s - Sample Application",
ApiVersion.szDescription);
//CWnd *CWinThread m_hWnd=m_dlgpropertysheet;

pApp->m_pMainWnd=m_dlgpropertysheet;(THIS WAS THE PROBLEM)
m_dlgpropertysheet->Create();