title: shortcut icon of QAction on qtoolbar disappeared after closing the triggered dialog
body: <p>I added a QAction to the QToolBar as a shortcut with an icon, when the icon is clicked(QAction emitts signal triggered()),an dialog will popup.The strange thing is when I close that dialog,both the icon on the QToolBar and QAction in the Qmenu are disappeared.Have anybody encountered this situation?
  <img src="https://i.stack.imgur.com/yCRfN.jpg" alt="Before clicking the first icon"></p>
  
  <p><img src="https://i.stack.imgur.com/yJ95V.jpg" alt="After closing the QDialog triggered by the action of the first icon"></p>
  
  <p><strong>Finally I find why.I delete a non-NULL pointer in the dialog!</strong></p>