I load my qt-dll by button clicking:
Qt Code: 
  1. void CMFCTestDlg::OnBnClickedButton1()
  2. {
  3.     HINSTANCE hinstLib; 
  4.     MYPROC ProcAdd; 
  5.     BOOL fFreeResult, fRunTimeLinkSuccess = FALSE; 
  6.  
  7.     // Get a handle to the DLL module.
  8.  
  9.     hinstLib = LoadLibrary(TEXT("qttest.dll")); 
  10.  
  11.     // If the handle is valid, try to get the function address.
  12.  
  13.     if (hinstLib != NULL) 
  14.     { 
  15.         ProcAdd = (MYPROC) GetProcAddress(hinstLib, "createWindow"); 
  16.  
  17.         // If the function address is valid, call the function.
  18.  
  19.         if (NULL != ProcAdd) 
  20.         {
  21.             fRunTimeLinkSuccess = TRUE;
  22. 			char * params[] = {"1"};
  23.             (ProcAdd) (1, params); 
  24.         }
  25.  
  26.         // Free the DLL module.
  27.  
  28.         fFreeResult = FreeLibrary(hinstLib); 
  29.     } 
  30.  
  31.     // If unable to call the DLL function, use an alternative.
  32.  
  33.     if (! fRunTimeLinkSuccess)
  34. 		MessageBox(L"Message printed from executable\n");
  35. }
To copy to clipboard, switch view to plain text mode 

Function createWindow() is as follows:
Qt Code: 
  1. extern "C" __declspec(dllexport) int createWindow(int argc, char *argv[])
  2. {
  3. 	new QApplication(argc, argv);
  4. 	Q_CHECK_PTR( qApp );
  5.  
  6.     MainWindow* w = MainWindow::getInstance();
  7. 	qApp->setMainWidget(w);
  8. 	w->show();
  9.  
  10. 	return qApp->exec();
  11. }
To copy to clipboard, switch view to plain text mode 

When I click button then MainWindow appears, I close that window and then I click button again -- raising up assert: qcoreapplication: there should be only one application object...

Then I wrote the following code:
Qt Code: 
  1. void MainWindow::closeEvent(QCloseEvent * event)
  2. {
  3. 	qApp->exit();
  4. 	delete qApp;
  5. }
To copy to clipboard, switch view to plain text mode 
But now i'm getting error in _CrtIsValidHeapPointer method after second closing of MainWindow.

How to unload qt-dll correctly?