wxGUI/gui_core: enable/disable toolbar long help if toolbar exists (#1922)

e.g. switch from 2D view -> Vector digitizer and then quit digitizer

gui/wxpython/gui_core/mapdisp.py

@@ -400,7 +400,8 @@ class MapFrameBase(wx.Frame):
     def StatusbarEnableLongHelp(self, enable=True):
         """Enable/disable toolbars long help"""
         for toolbar in six.itervalues(self.toolbars):
-            toolbar.EnableLongHelp(enable)
+            if toolbar:
+                toolbar.EnableLongHelp(enable)
 
     def ShowAllToolbars(self, show=True):
         if not show:  # hide