Fix to crash in QWindow::event when delete this called on closeEvent

Starting from Qt 5.11 QWindow::event is called after QDialog::closeEvent
which would cause a crash if "delete this" was called on closeEvent.  The commit
that changed this was e0b5ff4ad5. Added
a check before QWindow::event call utilizing QPointer to prevent the
function call in case object is destroyed by a user in close event handler.

Change-Id: I64a4a0f3271714e55bf7e806177f0d8b39b67fa3
Fixes: QTBUG-84222
Pick-to: 5.15 5.12
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qwidgetwindow.cpp

@@ -249,10 +249,14 @@ bool QWidgetWindow::event(QEvent *event)
     }
 
     switch (event->type()) {
-    case QEvent::Close:
+    case QEvent::Close: {
+        // The widget might be deleted in the close event handler.
+        QPointer<QObject> guard = this;
         handleCloseEvent(static_cast<QCloseEvent *>(event));
-        QWindow::event(event);
+        if (guard)
+            QWindow::event(event);
         return true;
+    }
 
     case QEvent::Enter:
     case QEvent::Leave:

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -419,6 +419,7 @@ private slots:
     void receivesLanguageChangeEvent();
     void receivesApplicationFontChangeEvent();
     void receivesApplicationPaletteChangeEvent();
+    void deleteWindowInCloseEvent();
 
 private:
     bool ensureScreenSize(int width, int height);
@@ -11774,5 +11775,23 @@ void tst_QWidget::receivesApplicationPaletteChangeEvent()
     QApplication::setPalette(origPalette);
 }
 
+class DeleteOnCloseEventWidget : public QWidget
+{
+protected:
+    virtual void closeEvent(QCloseEvent *e) override
+    {
+        e->accept();
+        delete this;
+    }
+};
+
+void tst_QWidget::deleteWindowInCloseEvent()
+{
+    // Just checking if closing this widget causes a crash
+    auto widget = new DeleteOnCloseEventWidget;
+    widget->close();
+    QVERIFY(true);
+}
+
 QTEST_MAIN(tst_QWidget)
 #include "tst_qwidget.moc"