XTFlat TabControl Assertion

I am using a CXTFlatTabCtrl to create a flat tab control
I have a class called
class CUserDockWnd :CUEXTPDockWindow

{

  CXTFlatTabCtrl m_objUserFlatCtrl;
  CUserFlexGrid m_objFlexGrid;
 
}
I have another class which is inherited from  CVsflexgridl1 (VS FlexGrid 8.0)

class CUserFlexGrid : public CVsflexgridl1
{

}


CUserDockWnd ::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
    if (CUEXTPDockWindow::OnCreate(lpCreateStruct) == INVALID_VALUE) {
        UESRCLINKTRACE(_T("CUserDockWnd ::OnCreate() Failed\n"));
        return -1;
    }     
    m_objUserFlatCtrl.Create(WS_CHILD | WS_VISIBLE | FTS_XT_HSCROLL | FTS_XT_HASARROWS | FTS_XT_BOTTOM,CRect(0,0,0,0), this, IDC_FLATTAB))
    m_objFlexGrid.Create(NULL,WS_CHILD, Rect, &m_objUserFlatCtrl, IDC_UE_GRID_MSGAREA, NULL, NULL, NULL)
SetChild(&m_objUserFlatCtrlb);

}


On Debug mode I am getting Assertion window like below



Assertion Is coming on the following funtion


int CXTExcelTabCtrl::InsertItem(int nItem, LPCTSTR lpszItem, CWnd* pWndControl)
{
    const int cItems = GetItemCount();
    if (nItem < 0 || nItem > cItems)
        return -1;

    if (pWndControl)
    {
        ASSERT(::IsWindow(pWndControl->m_hWnd));

        // managed view should be a hidden child
        ASSERT((pWndControl->GetStyle() & (WS_VISIBLE | WS_CHILD)) == WS_CHILD);

....
}