Pcbnew: fix crash on exit after using the track length tool.

This is due to the fact the PNS_TUNE_STATUS_POPUP was not hidden
before calling its destructor.
It is now fixed by setting its virtual destructor calling Hide().

TODO: see if it could be used for any class derived from STATUS_POPUP.
(We already have this issue in other places)

pcbnew/router/pns_tune_status_popup.h

@@ -41,6 +41,10 @@ public:
         STATUS_TEXT_POPUP( aParent )
     { }
     
+    // Ensure the widgets is hidden before deleting it
+    // Otherwise we have a crash
+    ~PNS_TUNE_STATUS_POPUP() override { Hide(); }
+
     void UpdateStatus( PNS::ROUTER* aRouter );
 };