shortcut icon of QAction on qtoolbar disappeared after closing the triggered dialog [closed]

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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?

Finally I find why.I delete a non-NULL pointer in the dialog!