Comment 1 for bug 1623044

Same here,
Traceback (most recent call last):
  File "/usr/share/gtg/GTG/gtk/plugins.py", line 254, in on_plugin_toggle
    self.pengine.activate_plugins([plugin])
  File "/usr/share/gtg/GTG/core/plugins/engine.py", line 221, in activate_plugins
    plugin.instance.activate(api)
  File "/usr/share/gtg/GTG/plugins/export/export.py", line 101, in activate
    self._preferences_apply()
  File "/usr/share/gtg/GTG/plugins/export/export.py", line 382, in _preferences_apply
    self.plugin_api.add_menu_item(self.menu_item)
  File "/usr/share/gtg/GTG/core/plugins/api.py", line 135, in add_menu_item
    widget.get_submenu().append(item)
AttributeError: 'NoneType' object has no attribute 'get_submenu'

Fixed for now but adding more times the item in menu:
#=== Changing the UI =========================================================
    def add_menu_item(self, item):
        """Adds a menu entry to the Plugin Menu of the Main Window
        (task browser).

        @param item: The gtk.MenuItem that is going to be added.
        """
        widget = self.__builder.get_object('plugin_mi')
        try:
            widget.get_submenu().append(item)
        except:
            pass
        else:
            widget.show_all()

Sorry, it's not a fix, only a workaround and now I can use the plugin :)