title: problem with QMainWindow setCentralWidget and stackedWidget
body: <p>i have gui the i created in the designer , simple one .<br>
  QMainWIndow that contains stackedWidget , the application starts with<br>
  stackedWidget index 0 that contains qwebkit widget and after some user flow ,it changes to stackedWidget<br>
  index 1 that contains QTree widget , to center the first widget i use in the QMainWindow constractor this line of code<br>
  this->setCentralWidget(ui.webView); but when the application switching to index number 1 im getting exception that is<br>
  coming from the switching command .<br>
  why ?  </p>