Do not crash when closing editor while diagnostic tool tip is shown

Make sure that the tool tip instance is reset to nullptr even if it was
closed automatically.

Change-Id: I3c718634d90dddc5822a634d082c263f51901684
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/libs/utils/tooltip/tooltip.h

@@ -30,6 +30,7 @@
 
 #include <QSharedPointer>
 #include <QObject>
+#include <QPointer>
 #include <QTimer>
 #include <QRect>
 
@@ -107,7 +108,7 @@ private:
     void showTip();
     void hideTipWithDelay();
 
-    Internal::QTipLabel *m_tip;
+    QPointer<Internal::QTipLabel> m_tip;
     QWidget *m_widget;
     QRect m_rect;
     QTimer m_showTimer;