I have simple MainWindow. MdiArea is in it. I can't create more small mdichild.
code of MainWindow constructor:
Qt Code: 
  1. MainWindow::MainWindow(QWidget *parent) :
  2.     QMainWindow(parent),
  3.     ui(new Ui::MainWindow)
  4. {
  5.     ui->setupUi(this);
  6.     mdiArea = new QMdiArea;
  7.     setCentralWidget(mdiArea);
  8.     MdiChild *one = CreateChild();
  9.     MdiChild *two = CreateChild();
  10.     mdiArea->addSubWindow(one);
  11.     mdiArea->addSubWindow(two);
  12.  
  13.     connect(ui->actionNew_Document,SIGNAL(triggered()),this,SLOT(CreateNewDocument()));
  14.  
  15.  
  16. }
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Qt Code: 
  1. MdiChild* MainWindow::CreateChild()
  2. {
  3.     MdiChild *newChild = new MdiChild;
  4.     static int count = 0;
  5.  
  6.     newChild->setWindowTitle(QString("%1 - %2").arg(count++).arg("Document"));
  7.  
  8.     return newChild;
  9.  
  10. }
  11. void MainWindow::CreateNewDocument()
  12. {
  13.     mdiArea->addSubWindow(CreateChild()); //This doesn't work
  14. }
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I commented "mdiArea->addSubWindow(CreateChild());" and added QMessageBox and that worked. so slot function is executed. but can't figure out why addSubWindow doesn't work.

Capture.JPG

When I press New Document action,new window doesn't appear.