Clear qt_tablet_target in ~QWidgetWindow.

Task-number: QTBUG-34007

Change-Id: If67563e5d9c4040256b58521773736e14db6fbaf
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/widgets/kernel/qwidgetwindow.cpp

@@ -86,6 +86,12 @@ QWidgetWindow::QWidgetWindow(QWidget *widget)
     connect(m_widget, &QObject::objectNameChanged, this, &QWidgetWindow::updateObjectName);
 }
 
+QWidgetWindow::~QWidgetWindow()
+{
+    if (m_widget == qt_tablet_target)
+        qt_tablet_target = 0;
+}
+
 #ifndef QT_NO_ACCESSIBILITY
 QAccessibleInterface *QWidgetWindow::accessibleRoot() const
 {

src/widgets/kernel/qwidgetwindow_qpa_p.h

@@ -58,6 +58,7 @@ class QWidgetWindow : public QWindow
     Q_OBJECT
 public:
     QWidgetWindow(QWidget *widget);
+    ~QWidgetWindow();
 
     QWidget *widget() const { return m_widget; }
 #ifndef QT_NO_ACCESSIBILITY