Fixed crash in QWizard running on old Windows systems.

When compiling the QWizard on a Windows system where
QT_NO_STYLE_WINDOWSVISTA is not set and running the app
on an old system that has no Vista support, the app would
crash in the paint event of the Vista Back button. In this
scenario, the Vista Back button is not needed (i.e. the
regular Back button next to the Next button is used), so
the fix is simply to avoid instanciating it altogether.

Reviewed-by: janarve
Task-number: 252662

3 files changed, 8 insertions, 3 deletions

diff --git a/src/gui/dialogs/qwizard.cpp b/src/gui/dialogs/qwizard.cpp
index 32395c4..298f23f 100644
--- a/src/gui/dialogs/qwizard.cpp
+++ b/src/gui/dialogs/qwizard.cpp
@@ -1465,7 +1465,7 @@ void QWizardPrivate::handleAeroStyleChange()
         return; // prevent recursion
     inHandleAeroStyleChange = true;
 
-    vistaHelper->backButton()->disconnect();
+    vistaHelper->disconnectBackButton();
     q->removeEventFilter(vistaHelper);
 
     if (isVistaThemeEnabled()) {
@@ -1491,7 +1491,7 @@ void QWizardPrivate::handleAeroStyleChange()
         q->setMouseTracking(true); // ### original value possibly different
         q->unsetCursor(); // ### ditto
         antiFlickerWidget->move(0, 0);
-        vistaHelper->backButton()->hide();
+        vistaHelper->hideBackButton();
         vistaHelper->setTitleBarIconAndCaptionVisible(true);
     }
 
diff --git a/src/gui/dialogs/qwizard_win.cpp b/src/gui/dialogs/qwizard_win.cpp
index 64696de..8aad4af 100644
--- a/src/gui/dialogs/qwizard_win.cpp
+++ b/src/gui/dialogs/qwizard_win.cpp
@@ -239,9 +239,11 @@ void QVistaBackButton::paintEvent(QPaintEvent *)
 QVistaHelper::QVistaHelper(QWizard *wizard)
     : pressed(false)
     , wizard(wizard)
+    , backButton_(0)
 {
     is_vista = resolveSymbols();
-    backButton_ = new QVistaBackButton(wizard);
+    if (is_vista)
+        backButton_ = new QVistaBackButton(wizard);
 }
 
 QVistaHelper::~QVistaHelper()
@@ -310,6 +312,7 @@ void QVistaHelper::drawTitleBar(QPainter *painter)
             QRect(0, 0, wizard->width(), titleBarSize() + topOffset()),
             painter->paintEngine()->getDC());
 
+    Q_ASSERT(backButton_);
     const int btnTop = backButton_->mapToParent(QPoint()).y();
     const int btnHeight = backButton_->size().height();
     const int verticalCenter = (btnTop + btnHeight / 2);
diff --git a/src/gui/dialogs/qwizard_win_p.h b/src/gui/dialogs/qwizard_win_p.h
index cbb3b17..148be26 100644
--- a/src/gui/dialogs/qwizard_win_p.h
+++ b/src/gui/dialogs/qwizard_win_p.h
@@ -94,6 +94,8 @@ public:
     void resizeEvent(QResizeEvent *event);
     void paintEvent(QPaintEvent *event);
     QVistaBackButton *backButton() const { return backButton_; }
+    void disconnectBackButton() { if (backButton_) backButton_->disconnect(); }
+    void hideBackButton() { if (backButton_) backButton_->hide(); }
     void setWindowPosHack();
     QColor basicWindowFrameColor();
     enum VistaState { VistaAero, VistaBasic, Classic, Dirty };