wxGUI/mapwin: check if 'BufferedMapWindow' instance exist before calling '_runUpdateMap()' method (#1456)

Related to switching MAPSETs between different LOCATIONs if the
3D view is activated.

gui/wxpython/mapwin/buffered.py

@@ -893,7 +893,7 @@ class BufferedMapWindow(MapWindowBase, Window):
                 return
 
     def _onUpdateMap(self, event):
-        if self.timerRunId == event.pid:
+        if self and self.timerRunId == event.pid:
             self._runUpdateMap()
 
     def _runUpdateMap(self):