I replace QWorkspace -> QMdiArea

Qt Code: 
  1. //workspace = new QWorkspace;
  2.     mdiArea = new QMdiArea;
  3.     //setCentralWidget( workspace );
  4.     setCentralWidget( mdiArea );
  5.  
  6.     createActions();
  7.     createMenus();
  8.     createToolbars();
  9.  
  10.     showScenePanel();
  11.     showControlPanel();
  12.  
  13.     scenePanel = new ScenePanel;
  14.     //scenePanel = qobject_cast<ScenePanel *>((workspace->windowList()).at(0));
  15.     scenePanel = qobject_cast<ScenePanel *>((mdiArea->subWindowList()).at(0));
  16.     controlPanel = new ControlPanel;
  17.     //controlPanel = qobject_cast<ControlPanel *>((workspace->windowList()).at(1));
  18.     controlPanel = qobject_cast<ControlPanel *>((mdiArea->subWindowList()).at(1));
  19.  
  20.    scenePanel->setWindowIcon(QIcon(":/Icons/Scene.png")); // <---- RED LINE
  21.     controlPanel->setWindowIcon(QIcon(":/Icons/Control.png"));
To copy to clipboard, switch view to plain text mode 

But the program crashes at run time (on the red line)