Fix access after delete in GTK2 dialog helpers

Each QGtk2*DialogHelper class owns a QGtk2Dialog. It is possible that
QGtk2Dialog's parent window is destroyed before QGtk2*DialogHelper.
Then the QGtk2Dialog is destroyed, because of the QWindow parent/child
relationship. Before destroying QGtk2*DialogHelper,
QQuickPlatformFileDialog calls QGtk2*DialogHelper::hide, which will
then operate on the already destroyed QGtk2Dialog.
  Break the parent/child relationship when QGtk2Dialog's parent is
destroyed.

Change-Id: Ie2bda234f759e7bf4d0642accd82fdc38e0379c2
Task-number: QTBUG-49203
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>

src/plugins/platformthemes/gtk2/qgtk2dialoghelpers.cpp

@@ -71,6 +71,9 @@ Q_SIGNALS:
 protected:
     static void onResponse(QGtk2Dialog *dialog, int response);
 
+private slots:
+    void onParentWindowDestroyed();
+
 private:
     GtkWidget *gtkWidget;
 };
@@ -108,6 +111,8 @@ void QGtk2Dialog::exec()
 
 bool QGtk2Dialog::show(Qt::WindowFlags flags, Qt::WindowModality modality, QWindow *parent)
 {
+    connect(parent, &QWindow::destroyed, this, &QGtk2Dialog::onParentWindowDestroyed,
+            Qt::UniqueConnection);
     setParent(parent);
     setFlags(flags);
     setModality(modality);
@@ -144,6 +149,12 @@ void QGtk2Dialog::onResponse(QGtk2Dialog *dialog, int response)
         emit dialog->reject();
 }
 
+void QGtk2Dialog::onParentWindowDestroyed()
+{
+    // The QGtk2*DialogHelper classes own this object. Make sure the parent doesn't delete it.
+    setParent(0);
+}
+
 QGtk2ColorDialogHelper::QGtk2ColorDialogHelper()
 {
     d.reset(new QGtk2Dialog(gtk_color_selection_dialog_new("")));